Because compared to our other positions our secondary isn't quite as strong.
Compare our other positions to CB/S. And by strong, I mean talented.
Is our secondary stronger than our LB corps? No
our DL? No
our OL? No
our WR/TEs No
QB? No
RB? No
It's not that our secondary is weak....but if you look at an area to attack as an opposing coach, you're going to say your best shot at success lies with your WRs going against our CBs.
I've never said their CBs are better than ours......I just think their 5 WR set gives us more match up problems than our 3WR/Witten set does for them.