In his answer, he cites a 1999 letter from the United States Conference of Catholic Bishops about the matter that says:
In liturgical rites, clerics shall wear the vesture prescribed in the proper liturgical books. Outside liturgical functions, a black suit and Roman collar are the usual attire for priests. The use of the cassock is at the discretion of the cleric.I was happy to see this. I was unaware of this norm and am glad to see it; the cassock is simply more comfortable than the suit.