I think it's because he had such interesting ideas, things that were extremely original, but also stories and ideas that readers and audiences can relate to in some way. Some of the best science fiction can be very dense; The Diamond Age, or Solaris. Even Roadside Picnic is difficult to film, because how do you visually depict something that's entirely beyond human understanding? (Tarkovsky gave it a helluva try though!) Dick is focusing more on the people; the science fiction setting is a way to get at his ideas about things like what defines humanity.

That's my take anyways.