So Starlight obviously wasn't a main character on the show back when the movie was being written, which is why she basically has a glorified background role in it. But by the time the second movie rolls around, they'll probably want to add her in a bigger role (in line with the fact that the show is basically Glimmy & Friends now), but it'll be extremely jarring though since she'll go from background pony in one movie to part of the main cast in the other. It'd confuse all the normies that don't watch the show.