With handhelds becoming virtually extinct, mobile games have largely filled that niche role of being places where side-games for franchises can be made.
The other issue is, most of those mobile games are dependent on a micro-transactional profit system, that actually makes the games very hollow and repetitious; make the menus and the 2-d/3-d art really nice looking, but only have so many assets, and hide a majority of content behind pay walls or play-walls that require either hundreds of dollars/luck or multiple months or years worth of dedication to unlock.
Oddly enough, I think most fans would've rejoiced to see a Xehanort based main-game, I don't think Nomura could swing it, considering he has these two other titles he's already getting funding for. Xehanort is the original Riku, in many ways, after all.
