Well PC gamers have been spoiled by, well, PC games. They want good multiplayer support (over LAN or internet) as well as good (realistic) graphics and multi-sourced sound. Fighting games don't really offer too much of any of those things. Most good Capcom and SNK games only have two player mode on the local machine and are 2D, a big no-no for PC games (PC gamers nowadays demand nothing but 3D). RE is also single-player only and has pretty crappy graphics, compared to PC standards.
They generally don't fare too well against the likes of PC games. Developers need to have good returns on investment (profits) and companies like Capcom generally do much better on arcade and consoles. Why port games to PC when there's only going to be minimal profits?
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