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 have 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 a lot but not the "in-universe rules". See Zoids for example. We see different Zoids titles, but they still stay consistent with the rule that there's an MC that has a Zoid as the Mech Pilot fighting other Zoid pilots. Yu-Gi-Oh battle with cards, and Pokemon with its concept of Gotta catch 'em all, battle with other trainers and wild Pokemon and compete in a league.
However, in Digimon, as a season/title ends, they make these makeovers in the story's scenarios and mechanics of the rules of battle.
For example, in the first season of the Digimon series (Digimon Adventure), we see 7 Chosen Ones who were 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. Nothing wrong there, then they throw in Frontier in which the humans transform into Digimons (as armor) and they were the ones fighting instead of the Digimons (Power Rangers style).
Also, in Digimon Adventure and Digimon Xros Wars, the "Chosen Ones" were having difficulties returning to the Real World, but the other titles can cross easily between worlds.
Is this because they were being called Pokemon's "rip-off"? Or is it a marketing strategy because like Pokemon, they also sell merchandise and trading cards? Is there any detailed explanation for this?