Simple: because that was a pretty lucky accident that probably is too risky to attempt to replicate.
Let me elaborate:
There are two comics that you should consider: How Obelix Fell into the Magic Potion When He Was a Little Boy Asterix and Obelix
All at Sea
The first one is a retelling that explains HOW Obelix felt in the cauldron when he was about 6.
The story is narrated by Asterix, apparently to the conventional readership, and tells that in childhood, Obelix was often bullied by other boys, until Asterix, to assist his courage, induced him to drink some of the magic potion that made the villagers invincible.
When they are interrupted in the act, Obelix falls into the cauldron containing the potion, and drinks it all , and is thereafter permanently under its influence.
As you can see, Obelix actually drinks the magic potion, not just "bathe" in it (so, sorry, no Achilles reference).
Now consider what happens in the second comic, Asterix and Obelix All at Sea.
In the story, Obelix actually manages to drink an entire cauldron full with the aforementioned potion.
The results?
First he gets turned into a stone statue, and then in a kid version of himself.
You may say that that effect was due to Obelix already having super-human strength when he drank the second cauldron...
but the same comic show the same destiny happening to Crustacius, a roman Admiral-
who never fell in a cauldron when young!
So, apparently, drinking too much magical potion in a single go may have dire consequences.
Based on the results, you may even say that it will have dire consequences most of the times !
This is somehow implied in multiple occasions during the series: every single time Obelix is given some magical potion (for example during the pyramid escape in Asterix & Cleopatra) Panoramix (or Getafix if you prefer his English name)
always try to give him as little potion as possible
(hinting he may already have feared that overdosing it may cause problems).
It is also worth noticing that even when Obelix originally fell in the magic potion that wasn't without side effects either: before the incident, Obelix is shown as being extremely pacifist .
While that personality trait may just originate from fear and the inability to defend himself, it is undoubted that he changes after gaining his characterizing strength.
Even his usual over-the-top reactions to being called "fat" - "WHO ARE YOU CALLING FAT" - seems to be something he developed after the incident.
At least I think we can assume Obelix appetite isn't a potion side-effect: based on the first comic, "How Obelix Fell into the Magic Potion When He Was a Little Boy", he was already somehow overweight before the incident
scifi.stackexchange.com/users/53529/BlueSoul
I have read all the Asterix comics, and most of them several times.
It is never stated directly why no others have followed Obelix's path, but there are some possible in-world explanations:
Obelix isn't very bright, and this could be a result of the overdose of strength potion, as could his obesity.
Few parents would want their children to become super-strong at the price of them becoming fat and stupid.
There may be some unknown long-term effects, such as sterility or early death from heart failure, and, following the principle of caution, parents refrain from exposing their children to this risk.
Apart from Asterix and Obelix, few of the Gaul villagers ever seem to leave the village's environs.
Since Getafix's potions generally are easily available in times of crisis, there is no need to dose up permanently, no more than there is a need today to wear combat armor all the time.
And then there is the obvious out-of-universe explanation: The stories wouldn't be fun if all the Gauls were super-strong all the time.
scifi.stackexchange.com/users/96722/Klaus Æ. Mogensen
TL;DR: The universe of Asterix is not meant to be consistent.
The author of the Asterix comics did not set out to create a logically consistent universe.
His writing lends itself more to parody and one-time problems the heroes have to face.
Our tendency as fans of a particular story (and by extension, its universe) is to seek logical truths that we can use to envisage all kinds of scenarios that the original author did not intend.
Very few authors start off by first creating the laws of a universe, and then creating its characters and dilemmas.
An easy example of this I can quote from Asterix itself: in one of the comics (It was one where Asterix and Obelix where in Rome, I cannot remember which) the simplest outcome would've been to storm Caesar's palace and take what they needed to complete the quest; Obelix tries this route, but Asterix stops him, saying "these guards are a lot tougher than what you're used to at home" (who can imagine anyone standing up to Obelix??).
This tells us the heroes have to face certain trials, even with the help of a potion that makes its user invincible.
I loved these comics when I read them as a child.
And the fact that you cannot always depend on your "invincibility potion", taught me a lot :)
scifi.stackexchange.com/users/112727/ChrisK
It's a moral thing.
Gauls are not to use the power of the potion for anything, but to defend themselves.
It mustn't be used to fight each other.
Getafix is very clear about that.
I think that is the answer.
And: it must be very annoying to have superpower in everydaylife.
Remember how often Obelix destroys doors and other stuff.
scifi.stackexchange.com/users/112686/BobBelcher1
The Gauls stood up against the Rome because they choose to give themselves and their children the freedom to live their life the way they see fit.
Forcing them to become eternal warriors would go against anything they could ever want for anyone, maybe the kid you just forced to become an abomination would have liked to become a bard but now he crushes any Instrument he touches.
scifi.stackexchange.com/users/35967/Dennis Christian
Given that the effects of Getafix's potion are permanent on Obelix since he fell in it as a child, why didn't the Gauls throw all of their children in the potion? By doing so, in about 15 years they would have an army of super-Gauls with which they could easily annihilate the Roman army.  Is there any in-universe explanation as to why this isn't an option?
Why aren't there more Gauls like Obelix?
scifi.stackexchange.com
<comics><asterix>
scifi.stackexchange.com/questions/206819
scifi.stackexchange.com/users/112640/Merry38
There is no clear reason why more Gauls did not bathe their children in the magic potion, but there are some plausible reasons that could be speculated. Firstly, it may simply be too risky, as there may be many associated side-effects, such as possibly sterility or herat-failure. Obelix is shown to not be too bright and overweight, both of which could be consequences of the initial potion overdose. Another reason could be simply that Gauls are simply not supposed to use the potion for anything other than defending themselves. There are a number of reasons for this, including the risks (e.g. Obelix is first turned into a stone statue and later becomes fat and rather limited intelligence), also there is a moral rule in that Getafix says the potion can only be used for self-defense, and finally the Gauls were resisting Rome so that they could be free to live their lives as they saw fit and not in order to become eternal warriors.
Because throwing a child into the potion is very risky. Even tho Obelix's gains huge strength he first gets turned into a stone statue and then into a child version of himself and later on in life he becomes docile and very touchy about people calling him fat. Obelix is not very bright and if that is a consequence of drinking the potion then maybe that is why other parents are wary of it. It is a moral question, as Getafix lays down the rule that the powers are only for defense and not to be used against fellow Gauls. The Gauls were standing up against Rome because they wanted freedom to live their lives as they saw fit - they did not want to become eternal warriors forever fighting.