The PC version is also the next gen console version. That's the main reason why it isn't out yet. They can't ruin the console launch by releasing it. That's if they have even finished the game yet.
Honestly, I feel they should have waited for two more months before releasing GTA V But I guess all the reviewers would be stating how the next-gen version is the best version, and the current consoles would hardly sell, causing more of a failure, than the people who will buy it again.
The problem here though is that delays typically don't sell as well because people who may have been interested and were on the fence already saw the game on YouTube and have less reason to purchase it.