Machines that were primarily popular in the US didn't get many demos.
Basically, the Apple II and CoCo.
I guess American coders were interested in programming something else.
The PAL people had something of a feedback loop with each demo released giving other coders new ideas and leading to new inventions, but this code was PAL and crapped out on NTSC so the American sceners never properly saw what was going on to learn from it. The C64 was popular in both regions but the NTSC demo scene there has always been... well, for want of a better word, more primitive
compared to what the PAL guys were doing at around the same time. It didn't stop them producing demos though...
I'm sure the Apple II's stone-age audio/visual capabilities have nothing to do with it.
Like Osgeld says, in demo coding circles that's usually just a challenge
- there's a lot of demos for the 48K Spectrum for example and that's only about twice the CPU grunt and a guaranteed means to latch onto the vertical blank away from what the Apple II has on the hardware front (usually with AY support but it's an add-on for the 48K machines like the Mockingboard is).
I have been threatening to do a demo for the apple II for years, and even have some routines scattered about on disks, but something usually takes priority
i've got a oneparter pretty much ready to go (well, i have to update the greets) which runs on 48K with a Mockingboard so i might have to push it out at some point soon... but i'm not placing any money on it working across the board though, it relies on a technique that only some of the documentation reckons will work from frame sync! =-)