They were recorded at the same time. The songs came from the same creative place. Hell, they even share a song between them ("Morning Bell"). So why does Amnesiac seem to always be dwarfed by the shadow of Kid A in comparison?
On Kid A, Thom Yorke uses his voice more as an instrument than as a vehicle for his lyrics. And it's a beautiful instrument -- mournful and keening one minute, pissed off the next, jubilant the song after that.