My guess is they did the market research and know Westerns don't do well outside of America. For instance when you look at the foreign figures for True Grit and Unforgiven then compare that to say Inception or Titanic and it's easy to see that Westerns do far better domestically than in foreign markets.
Combine that with the small market for people who don't own either console AND have a rig powerfull enough to run the game if it's anywhere near as demanding as GTA4 and a release would be pretty much pointless. Of course they could spend more time on the port and lower the system requirements - but that would just mean they'd need more sales to recoup the costs.
Latest Valve user survey shows 66% of user's PCs are set to English. When you take off the British, Australians and assorted others it's probably closer to 50% at best are US based.
So really their only big sales numbers would be coming from Americans who haven't or can't play it on a console - but have enough cash to buy a decent gaming rig.
Or in other words - it's a niche game on the PC and everybody would bitch about it being a console port anyway.