What Bad Books Have That Good Books Don't

What sells books?  

Its the question that haunts writers of all kinds.  Loads of great writing never gets published.  Loads of great published writing never gets popular.  And loads of horrible published writing becomes popular and turns into movies and the like.

Not fair, yes, but that's how it works.

What I have noticed, however, is that a lot of mediocre fiction gets published and gets fairly popular.  Us writers read said book and go, "Ugh!  The writing is horrible!", yet there are still millions of readers (many of whom don't read that much) that have picked up that same story and read it.

WHY?