It actually doesn't seem like a Bone movie series would be that difficult to do. The story is large, but not so overly complicated that it couldn't be condensed into something both simple and fun. But for some reason, no one has made it their life's mission to bring this family-friendly epic adventure to the big screen.
Nickelodeon gave it a shot in the 1990s, but it fell through before going into production. And based on what Bone creator Jeff Smith had to say about its adaptation ideas, we're probably better off without that one. Still, it seems a shame to leave it unadapted forever.