Because let’s face it, Adventure is the only iteration of Digimon people actually recognize. (Except for possibly Tamers)
 
Hell, Adventure is the reason why the franchise has re-emerged from obscurity.

@Sprite  
I don’t think you got it.

Toei please give Tamers love.

Because it takes a lot to create new characters, new situations, and even new digimon in a series that has always tried new things since its inception. Digimon has almost done it all, both in games, anime, and manga. This is its first time doing an actual reboot(for all I know) and I’m no board with it because they have tried new things before.

Because exploration and world building is a very good thing to focus on. Introduces many new characters, new problems, new character growth… You don’t want things to get overly complicated. The only anime that I know to some-what make use a multi-verse somewhat well is Yugioh ARC-V, since they were all integral to the plot of the series.  
Digimon mostly focuses on one or two universes, usually the Digital World and the Human World. We don’t really need any more than that.