Why so few new Amiga demos?
This is a plain question and I think it wasn't put out there to annoy the few who are active, but to find reasons (and maybe work to fix them??
). Well, Stingray and others have given you a few reasons why.
C64 scene is active.
Activity of one platform is perfectly unrelated to any other platform - "PC scene is active, why isn't the Mac scene?". If you're asking why coders are attracted to this platform over others, maybe it's because it's a) a fixed platform - easy to fight for the top and b) a more limited platform - which makes new effects an even more impressive feat - the coder who can overcome the limits gets much respect - but not so limited demos can't still sound and look cool. Compare with MSX (not fixed) or Spectrum (hard to make demos that have gfx you want to put on your wall or music you want to put on your mp3 player).
Reasons a and b for C-64 above is why I personally like to root for OCS ("1MB A500") as platform - it was the home computer everyone had, like the C-64.
Maybe also Stingray meant that it's not THAT inactive. There's quite a few prods released each year.