Because they're more athletic at every position.Why do AZ, UCLA, U$C, Ore keep beating us with 3-4 guys who are 6'9" + ? Tubelis, ballo, Bona, Etienne, Dante, Ware, Bittle - none of these have guard level skills.
Why can't we run 2 legit bigs AND more actual guards? We can lose some wings. I know Tad loves them, but FFS it isn't working.
Your better question would be why CU has owned Stanford despite them having so many NBA bigs while CU has pretty much only put its PGs (Alec, Spencer, Derrick, Kin) in the NBA along with a few forwards (Dre, Bey, Jabari).