Going from a single core CPU with multi-threading and all that jaz, to modern Quad Core CPUs, 64 bit opperating systems, and a bunch of other changes (like lack of hyperthreading/multithreading, which ever it is that modern cpus except the i7 line now re-includes) kinda tends to make older games a little harder to run.
Consoles are built with emulation of older games in mind. The Ps2 for example, basically had a PS1 in it.
The PS3 (orginally) had a ps2 in it, and has ps1 completely emulated by software. (its something like 10x the power to emulate one console on an other system. Running a PS2 game on a modern pc, is still extremely slow... At least it was for me...)
PC don't have BC hardware, its not hte same 'thing', pcs aren't just for gaming, they are for.. everything.
And adding in Old CPus, and Old ram, and old Operating systems, tend to make PCs less reliable, and other...t hings. (That and harder to atually BUILD and since we are all lazy/cheap S.O.B.s we'd rather have the easiest/cheapiest alternative, Specially when a few tweaks here and there by all our excellent coders out there for patching old games...