Abraham Riesman for Vulture:

In the real-estate bubble of the mid-aughts, when collateralized debt obligations were being passed around like joints at a Dead show and it seemed like the party would never end, the big banks did something unusual: they started investing in Hollywood.… [Post-crash] Faced with uncertainty and austerity, it became clear that it was only worth it to make movies if they either cost a pittance to conjure up or if they were big-budget spectacles that were sure to get butts in seats (and, in the wake of Avatar, could have tickets sold at a premium for 3-D screenings).

The crash made studios more risk-averse, and they were hungry for reliable tentpole flicks. Marvel was perfect for this new post-collapse film landscape.