PC's didn't have any dedicated sound hardware (unless you had a sound card), so any kind of sound/music playback would have to be done in real-time on the CPU,

PC's didn't have any dedicated sound hardware (unless you had a sound card), so any kind of sound/music playback would have to be done in real-time on the CPU, which didn't leave much for the rest of your game. C64, Amiga, etc. all had dedicated sound chips which could do a lot of the synthesis and mixing without burdening the CPU. (This is the same reason why PCs didn't get side-scrolling games until much later than consoles: consoles had dedicated hardware for scrolling tilemaps and displaying sprites, while PCs just had a memory-mapped framebuffer of pixels.)