That, and I think there's also another fundamental difference- kigurumi like that is basically trying to look like a completely animated character, where regular cosplay is often trying to recreate that character in real life.

Maybe that's the essential point right there. With regular cosplay, sure, you're done up as an anime character, but your friends can still tell it's you in the Rose Bride/Major Armstrong/whatever getup. You don't have that with kigurumi.