Bored on the way to STL, I read in the in-flight magazine yesterday that the "typical" cruising for the AA fleet includes
33,000 ft Boeing Super 80
37,000 ft Boeing 777
41,000 ft CRJ-700
It is this last one that mystifies me. Why does it fly at higher altitude?

(I realize that most posts to this board are about why we travel, not how, but I figure someone here can provide a simple answer.)