The animated TV series are popular, and the direct to DVD animated stuff seems to have been pretty succesful (at least for DC) so it seems odd to me that we haven't gotten any theatrically released animated superhero movies in ages.
I think animation has a lot of advantages over live action stuff, with the biggest one being budget. It would also open up the casting since we wouldn't have to worry about matching what people expect the character to look like. Now we could get our Will Smith Superman, or our Gina Torres Wonder Woman.
I love animation, and I would love to see a big budget theatrical animated DC or Marvel movie.
I'm sure that part of the reason for this is the way that film rights have been allocated.
For DC, anyway...multiple people held the rights to each of the characters depending on what format the character was to appear. Thus the fllm rights and TV rights might belong to different people. Typically when people but film rights they are doing so with the intention of making a live action movie since said films have the potential to make the most money. Thus anyone interested in animation was usually limited to TV.