Because sometimes you have to give all the spandex a break.
Thanks to Marvel?s current trend of planning all of their superhero franchises years in advance, audiences know exactly when every single one of their films will be hitting theaters for the next decade or so, barring any unforeseen circumstances.
The practical result of this is two-fold: it will become more difficult and embarrassing for Marvel to cancel any of their franchises that aren?t doing as well as they should be, and their actors can more easily plan around their responsibilities as superheroes.
Now that they know exactly when they?ll be filming each of their movies, they feel more comfortable taking on other projects that can be fit in between comic book outings. This allows them a unique opportunity to be part of a major film franchise (and rake in the big blockbuster bucks) while still feeling free to explore other types of movies and make sure that they?re diversifying their roles so that they don?t become typecast.
As a testament to this, Marvel actors are appearing in tons of different movies this year (which makes a lot of sense, considering exactly how many people populate that universe). And as jarring as it may be to see Thor starring as a mere mortal in another film, it?s undoubtedly a good thing for their careers to work as much as possible and not just ride out the superhero wave forever.