why do users vote for them to be demoted?

for your examples it's because skill based levels got (or at least perceived as) easier over time. newer technology affects skill based levels more than other types of levels, which makes them physically easier. also people are better at ship/wave than game modes like spider or swing (because ship/wave became very prominent in levels in general) so the community is inclined to find skill based levels easier over time.

ik that robtop or mods dont have a say in what tier the demon is and its the players that choose

mods have magnitudes more voting power than the players so I think the opposite of what you said is true.