Going back to this point, while the characters aren't as well known, I think in many cases those lesser known characters are a lot easier to adapt to film. The famous superheroes carry name recognition, but there's also a lot of problems that they bring, such as figuring out how to make their outrageous costumes work in live action.
Men in Black was a very obscure comic book, but it was also relatively easy to make into a film since it was basically guys in suits versus aliens. Hollywood has more experience with those ingredients than with men in pajamas.
I was just rereading Grendel last night and thinking what a great film that could be if you got someone as talented and committed to it as Guillermo Del Toro was to Hellboy...
A further problem is that, loyal as the hard-core fan base is, only a few (like Spiderman, etc.) are well known outside of core fan base.
The core fan base is the most critical of deviations from the source—and the filmmaker will find it almost impossible to satisfy both the core fans and a wider audience.
It's true. I initially got very excited and used to watch the Superman movies a lot. But I think they crossed the line with Hell Boy and League of Ext. Gent. was just retarded. They're making Superman, I think, and that should turn out to be interesting. That's some huge shoes they have to fill.