Well, it's about what the company want to promote. If they want Barry Allen as the main Flash in all media, no way Wally West is going to play that role anywhere (he's lucky to be still around
). One example that come to my mind is Duke Thomas. Scott Snyder created the character to be the next Robin and DC's higher ups were ok with it but the people at WB wanted to keep Damian as Robin (he was already everywhere outside of comics) and Duke had to find another identity. Apply this logic to SM/WW that for some years is something that WB/DC is promoting and then when they had to promote something else, they make it disappear everywhere.
I probably should've worded it differently.