I really think it's one of those incidents where other in-universe stuff is ignored to not screw up the story (i.e. to avoid permanent crossovers; series following their own canon; feel free to correct me on this).
Look at any other of the newer Marvel stories or movies (except "The Avengers"): They all contain some maniac antagonist causing lots of havok, potential casualities and maybe even some kind of apocalypse. And yet none of the other superheroes shows up. Are they just busy or too lazy? Once you introduce two "rescues the day" heroes to the same universe showing up in different comics, movies or animation series, you'll most likely cause situations not making any sense when seeing the universe as a whole. You'll have to keep the other heroes away without explaining it.
Or if you'd like another example from a different universe: Hercules and Xena. There've been several crossovers, especially with very strong enemies and stuff. However, there've been more than just one or two occassions where Hercules or Xena had to rescue the world without any help from each other (nothing mentioned, they probably didn't even try).