I think it's the combination of those cheap CPUs with 80s home computer hardware, where the hardware was designed as "game engine API", e.g. you just needed a handful of instructions to define a sprite and move it around on screen, without any library or driver inbetween. And those systems were hard-realtime. All timing was predictable down to the clock cycle, which allowed to synchronize rendering code with the video beam by cycle-counting.

These are things that are no longer possible on modern computers (but in return we gained a lot of performance by decoupling hardware components).