I think
@camaker may be much closer to accurate with this regard. Disney advertises the race but it's an uphill battle because Disney hasn't quite integrated Marvel the way it has other properties which means many who come for a 'Disney' experience may not enjoy the Avengers weekend. While the world has mostly accepted that Star Wars is owned by Disney and expect to see Star Wars at the Disney Parks. This is in part because Disney has made Star Wars events a huge focus for years even before officially acquiring the rights. Marvel on the other hand while having been under Disney's umbrella longer (2009 for Marvel, 2012 for Star Wars) has seen very little introduction in the way of events. This provides little incentive to come to Disney to experience Marvel properties. There are certainly many runners who are fans of Marvel and runners who are fans of Disney but Avengers weekend tries to fall into that small overlap while sidestepping the numerous legal boundaries that come with Marvel properties. Including but not limited to Marvel's limited rights to many characters and teams outside of the Avengers (Spider-man, Fantastic Four, X-Men, etc.), the continued existence of Marvel rides at Universal which prevents characters featured in them from being used, inability to mention Marvel in a ride name, etc.