From what I have seen, other than tachanun (nefilat apayim) the chazzan stands for the entire davening. Why is this so? Why not sit for things like krias shema?
tefila is b'makom korbanot (in place of the temple service. this is written all over the Tur in hilchot tefila) - and it was forbidden to sit in the temple except for Davidic Kings. we are lenient for the tzibur but not for chazan perhaps since he is our emmisarry and represents the tzibur.