It's hard to say just why 2D fighting crossover games exist. Well, actually, I think I know why they exist (hint: it starts with a $ and ends with another $).
Wow, a video game based on a documentary movie about penguins. Sure the movie was heartfelt and touching and the penguin is about as popular an animal there is right now.
Like anyone with a healthy sense of nostalgia, I look back on yesterday and wish that the games made by modern publishers could live up to the hopes, dreams, and ambitions of days gone by.