Actually fighting games have always been ported over to the pc... it's just up until the last few years, pc's weren't powerful enough to do a direct port (or perhaps I should say they didn't offer good enough gaming support).
Remember that the heyday of fighters was between 1991 and 1996, a time when computer games looked like crap. In that time capcom released everything from sf1 to ssf2t on the pc... all of which were crappy ports. Also In that time midway released all versions of mortal kombat on the pc... again all of which sucked. Snk was a "broke assed" company, they coudl barely afford to churn out the arcade games, which is they very reason that the neogeo home system was essentially the same, no added cost in porting. All the vf games were released on the pc. Namco couldn't port tekken because of an exclusive deal with sony.
So I'm not sure what you are talking about.
If you are referring to 3d games in the last few years or so the answer to that one is simple. To play a doa3+ or a mk eception on a pc.. it would have to be as powerful as the average pc required to play doom3 or battlefield 2. Gaming companies are probably betting that people are far less willing to upgrade for their favorite fighter, especially considering the console controller is better for fighters and online play is becoming available on the consoles.
Remember it wasn't until around 2000 that usb started becoming a standard and it wasn't until around 2k2-2k3 that cheap, "console-like" game controllers were available for the pc. In other words, you couldn't physically play the games on the pc. The mk and street fighter games wre hacked horribly to allow them to be playable with 4 buttons, which was the maximum number that the gameport allowed at the time. And one on one... forget it, you'v already used up all your buttons with the first player.
Guilty Gear is a very unpopular 2d fighter. They are only releasing the pc versions in Japan for the most part. The reasoning behind the pc port is simple..... there isn't an arcade version and the series is an unknown so they are trying to pimp the name. Mind you I like guilty gear, but I'm just saying.
Conclusion: Manufacturers tried to release pc ports of fighters for several years and for all their hard work they found that nobody wanted them in that form. So they've pretty much gave up at this point.