The simple answer is: money. When we started out we got really excited about the motivational value of the superhero themed routines and a created quite a few without realizing that we were breaking the trademark law.
Trademark is a name registered by a company. They, and they alone, can trade under it. They decide how that name can be used and, if they so choose, sell the rights to someone else in order to use it, too.
That’s exactly what happens with superhero names like Batman, Superman, Spiderman, Wonder Woman etc. Those are products and so are the names. Companies that can afford to buy the rights from Warner Bros and Disney to use the name, do. We simply can't afford it. Even though we don't sell anything here we still cannot, legally, use any of the names that have been trademarked.
So it is as simple as that, we can’t legally use any of the superhero names here anymore because they belong to someone else and we can’t afford to pay for the use.