Perhaps the issue is that, like so many science fiction authors, his ideas are interesting but his actual writing, characters and dialogue have nothing in particular to recommend them. In short story form, the ideas come and go quickly enough that one doesn't have a chance to get too bored with them. In a novel like Solaris, on the other hand, there's really only one idea and it takes a hell of a lot of words for it to actually get out there.

Asimov is similarly bad at actual writing, but his ideas are sufficiently good and they come sufficiently thick and fast even in his novels that he's still pretty okay to read.

In contrast an author like Neal Stephenson manages to write a science fiction story with proper characters who are interesting to read about even when they're not expounding on some science-fictional idea.