Very simply it will be down to a cost vs return analysis. If the developer / publisher deems the return to be big enough then it will appear on different platforms. If the costs are too great or the return to low (based on forecasted projections) then it will only appear on 1 or maybe 2 platforms. Most businesses regardless of sector work like this (certain products for certain markets etc). In the case of MK9 I imagine the developers looked at the analysis and came to the conclusion that porting it to the PC wasn't worth the cost considering the relatively small user base it would appeal to.
It would be interesting to see how it would work financially for DICE and the release of BF3. They're pumping obscene money into it, but working from the most capable platform and then scaling it down to the others makes the most amout of sense. Sadly this cannot be applied to all due to so many different factors; engine, coding, userbase, etc.
But then there are some, like Gears of War 2, that have no excuse. It was a Microsoft game (albeit under a different name), on a Microsoft console. The 360 and windows 7 gaming environments are so similar it's practically a drag 'n drop - and apart from anything else, it's Microsoft. PCs are their turf. Just...unforgivable. I'm convinced there are no humans working at Microsoft, and it's all run by one buggy, irrational, half-broken AI that hates us all.
Sort of, but all of my friends who game on pc have a controller. The wired 360 controller is one of the most needed peripherals as of late. Most PC controllers sucked. Got a lot of play time in on SF4. On the other hand no one I know has a keyboard and mouse they use on their consoles. They probably didn't bother as the first Gears on PC had a lot of issues (actually come to think of it, it was all pretty much GFWL). My copy ran fine though. No one played it on PC as I could never find a multiplayer game.
Sometimes the costs of doing so just mean it's not worth it. If a game won't sell very well then there's little point. It's not just the cost of porting it, you've got to support it, you've got to distribute it, many other costs involved too. So a developer will look at a game, and decide whether it will sell well enough on PC to make it worth the time, money and effort to port it.
Every time someone mentions that Gears of War 2 isn't on PC, someone else goes on a murderous rampage. Grr. I absolutely loved GoW. I'm not going to buy the crappy pice of w*nk that is the Xbox so I can play 2 and 3 in super crap res-o-vision, using a controller that I'd do better with if I used my feet. Sorry, got a bit carried away . Just hits a nerve, that particular issue.
The strangest case of this is Red Dead Redemption. Rockstar always usually release their games on PC as well as consoles, but for some reason didn't see fit to have a PC version of it. This is a game that I can't see how it wouldn't have sold well on PC.
It would cost hundreds of thousands (if not millions) to port to PC and get the retailers to stock it. They wouldn't sell that many copies as gamers who are "into" that sort of game will have a console (Generalisation) Answer = Maths
