What inhibit's Unix based OS's from running games well?

It's mostly market.

Direct X at the moment has better development tools than OpenGL (but good ones are now being made), so games studios like to use it even though OpenGL is a much more versatile graphics API.

Drivers are a third issue. Intel has good open source drivers, but intel doesn't have good graphics units. Nvidia and AMD have decent proprietary drivers and lower performance opens source drivers. With proprietary drivers things can be hard to update and get working properly, but the open source drivers just don't have the same performance. Fortunately the open source drivers are getting better all the time, and AMD and NVidia are working even harder to keep their drivers up to date and tracking the newest Linux developments as a result of the steam box.

Lately you've seen games like Team Fortress 2, DOTA 2, Portal (soon Portal 2), and Left for Dead ported to Linux. And in the past Tournament 3 had a Linux version on the retail cd.

But going back to the market, most AAA games don't bother porting because they have custom engines or resources that would take a lot of effort to port, wheras a lot of indie games are on platforms or engines that have already been ported.

Another big issue in the past was middleware. Things that hosted multiplayer rooms or maybe the physics engine ... charges a per platform licence as well as a per box. Some wouldn't allow porting away from windows at all. The steam-works API partially solves this problem, but it's still an issue with some games.

OpenGL also allows access to the gpu, and historically has been faster then DirectX. DirectX is made by Microsoft and retains the Microsoft bloated managed mentality. OpenGL is open source, works well on Linux, and is lean and mean and much faster. And this has absolutely nothing to do with what games run with Windows, and what games run with Linux.

There are many games that run on linux, some through wine, and some natively. Those that run natively run very well, are very stable, and are often faster than the Windows equivalent.

It's all about the money, honey. If I make my game for Windows, how much money will I make? If I make it for Linux, how much money will I make? Most game makers are in it for the money, so they choose windows.

This is changing. Steam is out for Linux, and more and more games are coming out for Linux. Windows, as the top gaming platform, is coming to an end. The future of gaming belongs to Linux.

Game

I do not know where you get your information from. Linux runs games extremely well (aside from the mac OS (OSX) which has 2GB of overhead only leaving programs 2GB to work with). It has to do with market. If 93% of users use Windows, 4% use Linux, and 3% use OSX (yes, more people use linux then OSX) then you would make more money by targeting windows. Windows certiantly does not run games as smoothly as Linux. It just has more users.

Steam now supports linux. Games are starting to be developed for linux. You should try some of the free ones out.

Biggest Market = Biggest Gain

There is nothing that inhibits other OS's to run games... the makers are in it for income. Steam is now working its way into the Linux platform... and runs quite well.

One benefit that Linux has over Windows is that there is no third-party anti-virus bloatware running in the background gobbling up resources that the game could use.