Considering there are only 32 starting quarterbacks in the league, it's easy to see why they would statistically bust the most often. Every other position (except Center I guess) goes at LEAST 2 deep before you're considered a bust.
Sums it all up. No one takes centers in the first for the most part either.
OL and DT in particular also have a high bust factor because of the nature of the positions. Big guys are far more likely to get hurt and also the nature of the size they need leads to weight issues being more likely.
WR must also be up there, especially recently. That one is harder to explain, though I guess they are the position most dependent on the rest of their team. With poor OL and QB play they will find it harder.
Safety and LB being the lowest is not shocking. LB has 3, sometimes 5, positions you can play. Some can even move to safety. It is also one of the positions that changes the least and also seems to get the least respect in drafts. If you arent a perfect prospect, you arent a 1st round talent.
Same deal with safety. If you arent a perfect prospect you arent sniffing the first round. It also pretty much translates from college to the pros. The scheme wont change your responsibilities much. Another big thing for both positions is they will make up the majority of special teams. If you can contribute there you get a little longer to develop elsewhere.