I think, due to the rather modest success of the console-exclusive Revolver (920k LTD in the US, but that's across six years), a PC port wasn't planned at all initially and the immense success of the game caught Take-Two off-guard; with Redemption being followed by LA Noire (somewhat), Max Payne 3 and GTAV, there was no room to slot in a PC port amongst the work that the various R* studios were already doing, so it was put on the backburner.
The game sold tremendously well, PC gamers have been clamouring for a port for years, and belated PC ports of popular titles have proven to be a smart business move for other publishers (e.g. Alan Wake and Mortal Kombat 9). It's going to happen eventually.