So my best guess is that Rockstar was waiting on the "next-gen" assets before they bothered releasing Grand Theft Auto V on the PC. The game will be released this fall, alongside Xbox One and PlayStation 4 ports. They do not mention distribution platforms, but Steam is a fairly safe assumption, at least now that Games for Windows has been given its final rest.
Hopefully, this delay in releasing a PC version will be a temporary hiccup due to the overlapping console generations. With Grand Theft Auto IV, the same could not be said. The problem is, with how secretive Rockstar is, we cannot really tell whether the above assumption is true, or whether they were just non-committal to the PC platform until now. At either rate, until the PC version is launched, Rockstar has not and will not get my money. Of course, there is always that danger that, by the time the game does launch, I will not be able to afford its time or expense.
That's why you should always release the PC version as early as possible.