The reason is the (unofficial) law of emulation, and also why there isn't a Gamecube or a Saturn emulator. Your compulter, in all odds, has one measly little 32-bit processor. A PS2, as you guessed right, is too many bits for a little processor to handle. But there's another reason that explains it better. The law of emulation states that to emulate something, you need something at least 10x better, 20x better for flawless emulation. Ever wonder why ZSNES requires a 400mhz machine, or an NES requires a 200hmz machine? Every wonder why a PSX emulator can tax your system to the max?
Bascially, an emulator emulates all the chips in a console. While the process is simplified, it still requires alot of power to run right.
The X-BOX emulators, if they wen't by that approach, won't be coming out until at least 2 years down the road, when AMD get's their 64-bit chips past 4GHz or something

But an XBOX is different. It is based on the same archetecture as a x86 computer. So, in practicallity a WINE-like emulator is possible. This means a smaller CPU can be used, but that development can take a bit loinger. Another problem comes that the OS in an XBOX is much more efficient than Win 2000 on a PC. Reason, of course, is that an X-BOX doesn't have to start loading a whole bunch of DLLs or programs or desktops or whatever else, it just has to start a HDD, and run whatever game you need. So you'd still need a computer about 2x as fast right there.
There is a good X-BOX project started, at
http://www.caustik.com/cxbx/ . There is a new arrival supposedly able to play Halo, at
http://www.zophar.net/xbox.html called Xeon. Requires a full DirectX 9 card though, like the Radeon 9600 or the FX5600. It also requires WinXP, so the specs are a bit higher than that of CxBx.
Another reason? The X-BOX emulation scene just started.

In emulation time that is.
-edit- Oh, and you may be happy to hear that the PS2 emuilators are just starting to play commercial games now.