When No. 2 Missouri and No. 2 Duke lost in the N.C.A.A. tournament on Friday, the predictable themes of parity in college basketball reappeared. They disappeared as quickly over the weekend, however, as only two teams from outside the six power conferences advanced to the Round of 16. That is the fewest since 2003, meaning that the darlings have mostly left the field to the sport’s boldest brand names.