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?
one possible reason is that 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.
another reason could be out of respect. Like we find that it was forbidden to sit in the temple area except for a king. So too, it is not proper for a servant of a king to address his king while sitting down. all the more so for adressing the King of kings. hard to find a source for this because it is obvious.