athleticism is not - and it takes great athletes (along with great effort) to play really great defense. It's easier to play stifling in your face defense when you have better overall athleticism.
How many long and lean athletic defenders does BYU have on its team? I don't really see any. I know nothing about Wichita State's team this year, but I'm willing to bet they have several long 6'6"-6'8" athletic guys that are nothing like the defenders that BYU can put on the court.