I've heard from a few other writers who have this experience: it is hard to finish a book anymore. For me, I know at least three contributing reasons.
1) High standards/see behind the curtain. A lot of of what gets published would get ripped apart in critique group. This is a good lesson for critique group, actually: it doesn't have to be perfect, and "the rules" are really only guidelines. That said, the newer of an author someone is, the less they can get away with. More often for me is that it is all just so transparent. I see the whiff of death, I see how they're setting up the stakes, I see how their making the protagonist likable (save the cat). This is even worse in movies but there I find it an interesting puzzle. When I'm reading, I want to fall into the book!
2) Lack of time. Any time I'm reading, I'm not writing. However, the first rule of writing is that you're supposed to read a lot of books. A lot. Basically, those few people who write a first novel that just happens to be brilliant, or the larger set who write without planning, are those who have absorbed how to tell a good story without being taught, and mostly this happens through reading. So I strive to read more and I don't consider it an excuse that I'm busy, but I'm not going to waste time finishing a book that I don't find compelling.
3) Buying books for research, not pleasure. The corollarly of the 'read a lot' rule is to read a lot in your genre. On my bedstand, there are currently four novels with themes or setting involving the moon. When I talk to an agent, I need to know how to compare my book to those that are out there. Moon or earth near-future. My dirty little secret is also that I don't spontaneously read speculative fiction. Growing up, I read the classics that big fans of the genre suggested to me, but I don't have deep knowledge. So although I've found authors I like, it requires wading through a lot that I don't.
Up next: a book I was compelled to finish, despite it's flaws.