Actually QBs, RBs, LTs and WRs taken in the first five have a very high bust ratio. Its like 50%. CBs are usually taken outside of the first 5; and they have a much lower bust factor. I think thats because CBs are out there one on one mostly with WRs and you can see what they do and how they do it. Unlike LTs who are on the line and many factors can clowd just how good or bad they really are; and same for college QBs and RBs and WRs. CBs are not dependent on QBs to throw them the ball like WRs; or QBs to have good blocking, coaching, WRs, etc; or RBs to have good blocking.
If a CB has the speed and quickness, and shows an ability to anticipate and read QBs, and react well to the moves of the WRs, then the bust factor risk is much lower because you can easily SEE all that without anything else getting in the way.