Rockstar released a comment pertaining to the delayed release on the PC:
Source
Those two extra months better be worth the extra time to polish and optimize!
We're glad to see so many of you are excited for the upcoming release of the new versions of GTAV and we look forward to sharing more details with you soon. We are also incredibly excited to be bringing GTAV to the PC, but the game requires a little more development time in order to ensure that it is as amazing and polished as possible. Please do stay tuned as we reveal new features and information about all the new versions in the weeks ahead.
Source
Those two extra months better be worth the extra time to polish and optimize!