Speculation...
Angels were each aware of the perfect reality of God, and followed Satan in the sin of pride--that is, wholly understanding who God is, yet rejecting Him as God. Satan/demons understood it all, but rejected it. What more could be shown to them to convert them?
Humans, on the other hand, have been tempted to sin thru deceit. Evil often masquerades as good. So Christ is the prism that separates our colors.
Isa 28:16 "See, I lay a stone in Zion, a tested stone, a precious cornerstone for a sure foundation; the one who trusts will never be dismayed.
17 I will make justice the measuring line and righteousness the plumb line;
hail will sweep away your refuge, the lie, and water will overflow your hiding place.bebu