Why isn't Digimon consistent with its rule mechanics per story/season title? After reading this review about Digimon:

While the general basis of relationships between the show's kids and their digital companions remained constant over the course of the series, almost everything else went through many makeovers.
Some seasons feature seven human visitors to DigiWorld; others,
as few as three. In early episodes, the kids can't get back home; later stories show a new cast of youngsters effortlessly
journeying between the two worlds. And while in most stories each
kid has his own specialized Digimon sidekick, a full season of
episodes featured humans who could transform into the digital
beings rather than just summon them for battles.
Youngsters will certainly find this lack of continuity confusing, and
parents may be wary of introducing their kids to yet another marketing
conglomerate looking to rival Pokemon. (Digimon boasts a similar
assortment of tie-in video games, trading cards, virtual pets, and
other toys.) And it's worth noting that although the show's violence
is mostly of the flashy, fantasy variety and battles are reserved for
the digital beings, those who are seriously injured dissolve into
pieces that are said to regroup and reincarnate at a later date.

Anime titles that has certain rules like Pokemon's battling system, Bakugan, B-daman and Yu-Gi-Oh's Trading card Game. With every season or Arc those anime change the concept and lot but not the &quot;in-universe rules&quot;. See Zoids for example,
we see different Zoid Titles but still stays consistent with the rule that there's an MC that has a Zoid as the Mech Pilot fighting other zoid pilots. Yugioh battle with cards. and Pokemon with its concept of Gotta catch them all, battle with other trainers and wild pokemon and compete in a league.
However in Digimon, as a season / title ends, they make these Make Overs in the story's scenarios and mechanics of the rules of battle.
for example, in the first season of Digimon (Digimon Adventure 00) We see 7 Chosen Ones who where transported to the Digital World. In there they meet their seven companion Digimons (one partner for each).
The Second season, continues the first with the 2 veterans and 3 new Digi-destined. Nothings wrong there then they throw in Frontier in which the Humans transform in Digimons (as armor) and they were the ones fighting instead of digimons (Power Rangers style).
Also in Digimon adventure and Digimon Xros Wars the &quot;Chosen Ones&quot; were having difficulties returning to the Real World, but the other titles can cross easily between worlds right?
Is this because they were being called Pokemon's &quot;Rip-off&quot; ? or is it marketing strategy because like pokemon they also sell merchandise and Trading Cards.
Are there any detailed explanation for this?
I am incapable of giving a very good answer for this question, but I will try my best. As I have mentioned in my comment, the Digimon series is rebooted every title except for the Adventure series. The reason for this is most likely due to the fact that they feature a ageing cast instead of a Sazae-san type of show, where the characters never age, Pokemon falls into this category, so they follow Ash around for nearly 20 years.

After a little more thought, there is actually a very logical explanation for why Digimon was rebooted every time, it is mainly to keep it fun. For example, Pokemon has kept Ash at 10 so it is relatable to the children of every generation and thus keep it running for years and years, however, the defect is that Ash can never mature beyond a certain extent. Introduce Ash to new region, tell same story.

In Digimon's case, it is different, in every series, a large world threatening evil appears and the children grow and mature into near adults in their journey that spans the entire Digital World, the entire part is important as that means they are exhaustive, unlike Pokemon, which can just give a new region. Therefore, if it was Taichi and his group just going around defeating world threatening evil in a new region as they age, how horribly boring and awkward will that feel. Imagine a 20 year old jumping around shouting: "Me and Agumon will solve this with friendship!"

Therefore, to keep the Digimon series interesting while keeping the part of a world threatening evil and a journey that covers the entire world still existent and thus attract its audience, innovation is needed, so it is rebooted.

Therefore, with every series, they only thing that remains the same is 


Main character: Stereotypical Hot-blooded Shounen Guy
Problem is solved through friendship and all that other rubbish
Main Digimon is also a Stereotypical Hot-blooded Shounen Guy that follows main character's ideals


With every reboot, a new creation mythos is made for the show, therefore, each series features a different Digital World with its own unique origins. The traversing part about between the worlds in the shows is actually fixed, the main thing is you need a key or a great enough power in the form of a portal is required to traverse between the worlds.


Adventure 01: Portals
Adventure 02: D-3 Digivice + digital platform such as a computer = Portal
Tamers: Portals
Frontier: Trains + Portals
Savers: Portals
Xros Wars: Xros Loader + Power = Portal


The thing about Frontier is different  only in the form of having Humans as the Digimon instead but this is most likely to be an innovation for the show in order to attract a larger group of audience, a cheap market trick.

One more thing, the concept of the DigiDestined is not present in the other series, it is only in Adventure that this concept exists as a plot device. The children in the other series is still referred as such mostly in the west only. Therefore, the point is do not think the other main characters are "chosen" like Taichi and his group. They are just a bunch of random  shounen-esque children that had a meeting with the Digimon that forever changed their lives.