"learn on the job" situation. We need an experienced DC, preferably one that has been highly successful at the P5 level or at the very least the G5 level. Experience in a particular role matters...a lot.
Is this to say Nua couldn't kill it as DC? No, he very well could. It's just risky, and going into the Big 12 next season, it doesn't seem like BYU is in a position to take a risk like that.