I happen to be the webmaster (really, sysadmin, but why quibble) of a popular Mortal Kombat fansite.
In the first "generation" of Mortal Kombat (MK 1-4), every one of the fighting games made their way to PC. As soon as the first of the second "generation" games was announced (Mortal Kombat: Deadly Alliance), we were bombarded with people asking about a PC version, because only console versions had been mentioned. We asked reps from Midway (including a head of marketing) at E3, and the reps told us they had no plans to make a PC version, as they had never made any money on the PC versions.
Between E3 and the game's actual release, the questions regarding the availability of a PC version only intensified, and many folks were irate that there were no PC versions. Then we got to thinking about it... we knew a lot of people had PC versions, but there was no money made on it? We knew the games had no DRM, so it wasn't hard for us to believe that a good chunk of the people who had gotten the PC version before (and were asking for it now) were pirates. In fact, one of our staff members actually caught out someone demanding a PC version admitting he pirated other game software...
So, yeah, I can say that the entire lack of MK for PC now is due to piracy. The earlier games got pirated all to hell and they never made money off those ports, so why bother porting if it's not worth the money?