Last time I checked, I could still play Commander Keen and Wolfenstein 3D on my PC.
Yeah, that's almost 20 years.
Really, there are to my knowledge two primary reasons for an old game not working on a modern PC:
1) Sloppy programming. Whether it's a case of the programmers simply not giving a shit, or an attempt to optimize the code so well for the hardware of the time that it somehow clashes with modern hardware.
2) Abandoned standards. The previously mentioned IPX/SPX networking protocol, for example, is not used anymore as it hasn't served a purpose for about 10 years. No network cards support it anymore, just like no current graphics cards support 3dfx OpenGL.