Fantasy and SF novels have to do a lot more than mysteries do -- not just setting up plots and characters, but entire worlds, societies, and laws of nature -- so it makes sense that they'd be longer.

The shorter SF books of Ye Olde Days were just trying to ape other genres' lengths without thinking about what really made sense, with the result being that the books were all way too short and felt like plot outlines in book form and had to mostly drop characterization anyway.