My theory continues to be there are more offensive guys to go around than there are defensive guys. So the Power 5 schools can (generally) find enough of both to be fairly balanced but the G5 teams are left to make due with what's left which is more offense than defense.
I might be completely pulling this out of my @$$ but it seems to make sense and I'm going to go with it until the Chin proves me wrong (hopefully never).
You can scheme around issues much easier on the offensive side than the defensive side, due to having the ball and the advantages therein. What happens then at lower tier schools is they become offensive heavy to make up for recruiting disadvantages. Even if you're the chief bigwig school at a lower level, your ability to recruit superior athletes compared to your rivals is limited because you're all using the same pool left over after all of the upper divisions are done. The lower the division, the more this becomes the case.