Because at the time, the Alliance of 4 races was fighting the Goa'uld.
They wouldn't necessarily have the spare resources to fend them off and go chase after one lone planet that had rebelled a while back.
It's unclear when the Alliance ceased, but I assume by then, there was little reason to take over Earth and other planets that had fought off Goa'uld rule.
This explains why SG-1 were able to find so many diverse civilizations in their explorations.
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The Goa'uld believed the Ancients were protecting Earth This very question was addressed in the 2013 SG-1 novel Moebius Squared , which explores the altered past of Season 8's Moebius two parter.
The novels are considered to be in official continuity with the TV show.
* Daniel explains that he found records of other human uprisings aside from Egypt, such as one in Japan against a Goau'ld subservient to Ra.
Most of these rebellions had the direct aid of the Ancients (and their immediate descendants), who were refugees from Atlantis.
After many such uprisings and the one that finally drove out Ra from Egypt, he decided to stay away to avoid taking further losses at the hands of the Ancients.
By that point, the Goa'uld had enough humans on other worlds that they no longer required a supply from Earth.
In the events of the book itself, In 2800 B.C., a symbiote is stolen from Abydos so that Teal'c from the Moebius timeline can live, and Ra realizes the thieves are from Earth.
As punishment, he takes his mothership to Earth and invades once again.
Ra captures O'Neill and assumes he's an Ancient since he has the Ancient gene, which he is concerned about since Myrdin (Merlin) aided in the rebellion against him before.
Ra assumes that the Ancients are going to try to drive him off the planet again, yet believes they are in decline and no longer have the ability to protect Earth.
By the end of the book, O'Neill devises a plan in which three puddle jumpers cloak and decloak in order to pretend that they have an entire armada.
With the help of the Tok'ra queen Egeria, Ra is incapacitated during the battle and is led to believe that the Ancients were more powerful than he thought.
Ultimately, Ra decides that having underestimated the strength of the Ancients, and being driven off once again, he ought to leave Earth alone for good.
The Goa'uld had enough humans on other worlds, so there is no strategic reason to invade again.
* Author Jo Graham, who wrote many of the Stargate novels, stated that because every word of the novels was extensively reviewed by MGM and intended to integrate with the show, the novels are canon , although they had the right to disregard them in the future if they like.
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Back then it was just some backwater planet, Ra had already mined what Naquadah that was on Earth, So when they rebelled and buried the gate he just cut his losses and never bothered to go back.
Giving Earth time to develop on it's own.
Also as seen in SG:
Atlantis the ancients spend time there in the middle ages (Merlin)
so they might have shielded the Earth from any other Goa'uld who might have had their eye on it.
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First of all, the Goa'uld seem to have taken over the galaxy around 5000 BC.
Certainly, it hadn't been for any appreciable amount of time.
I think it's not inconceivable that while the Goa'uld had stargate travel, they had little to no spaceflight capacity 5000 years ago.
Specifically, it is stated in the finale of season 1, the Goa'uld took years to get from one system to another.
Given that timescale, it's no wonder they wouldn't want to attack Earth.
Further more, they likely didn't have as strong as an army as they clearly possessed in the time of the movie and SG-1.
Therefore, I don't think it's inconceivable that they couldn't attack Earth from space.
For reasons listed in this question , they couldn't use the Beta Gate either.
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By the time of the Rebellion on Earth there were probably hundreds of worlds populated by human slaves lorded over by hundreds of System Lords all vying for power.
Earth was also Ra's personal domain, so to poach humans on his world probably would be frowned upon violently,
only the most powerful or desperate Goa'uld would attempt such an affront.
At that point it was no longer a vital resource, with comparably poor mineral resources, and no Naquadah, it lost all strategic significance.
The conflict with the other 4 Alliance races, plus any internal fighting would make it a non priority for years.
And after a while it is just an embarrassment to Ra.
By the time he was no longer occupied elsewhere to deal with it it would have been a blow to his prestige.
Better to be forgotten and covered up.
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Why didn't the Goa'uld simply send a ship to Earth after Earth's rebellion?  This came up after looking at the following question: " why they didn't attack Earth through the Beta Gate ".
Why did the Goa'uld not send ships to attack Earth after Earth's rebellion?
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<stargate><stargate-movie>
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The Goa'uld believed the Ancients were protecting Earth.  This very question was addressed in the 2013 SG-1 novel  Moebius Squared , which explores the altered past of Season 8's  Moebius  two-parter. At that point it was no longer a vital resource, with comparably poor mineral resources, and no Naquadah, it lost all strategic significance. In addition, at that time, the Alliance of 4 Races was fighting the Goa'uld. The Goa'uld had other wars to fight and believed the Ancients were protecting the Earth. Moreover, the Earth was a minor backwater and not worth their attention as it no longer had vital resources.
The Goa'uld were fighting other wars The Goa'uld believed the Ancients were protecting the Earth. The Earth was a minor backwater not worth the attention, with no vital resources.