Re: adam w./mary jane w.
Because Spider-Man is a nerd fantasy. The entire world is built on the fantasy that you can transcend bullying and unpopularity by becoming a superhero. It revolves around him. You were written to be attached to Parker. Destined even. And, appearing through some door isn't going to change that.
Flash doesn't have to be some running joke. If he tries hard enough and finds people who value him for who he is, he can be something real.