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.