Film and television are businesses whose profits depend on the audience's interest. So the importance of a character in-story has always been a function of that character's popularity with the audience. If a character starts out less important but the audience goes crazy for them, then the creators will make that character more important in response. That's how Chief O'Brien went from nameless bit player to one of the main cast members of a spinoff series, and how Rom went from a nameless Ferengi pit boss to the Grand Nagus.
Heck, that's kinda how Picard became so central. The original idea behind TNG was that Riker would be the featured action/romantic lead while Picard was the elder statesman who stayed behind on the ship. But Patrick Stewart was far more popular than Jonathan Frakes, and he pushed for more action, so he ended up being the action lead of the movies instead of Riker. It's all about popularity. Data was the second-most popular character after Picard, so he was made the second-most featured character in the movies.
