Easy, base the movie around Megas. If there is over 30 Megas then we have over 100 'pokemon' Since GameFreak considers megas new pokemon. That is probably why they didn't make so many. But not really an excuse to not have a Region Trio and Pseudo Legend line, unless one of those pokemon we have seen ends up being a Pseudo legend some how.
I didn't like the Mega Evolutions to begin with so it's even more upsetting to hear they're cutting out things to make them the main focus of the game...
I'd personally rather have evolutions than a separate form of the same Pokemon. But I guess my single opinion doesn't matter.