There is a reason Defensemen don't go first overall that often. They're not unimportant but they aren't (on an individual basis) really gamebreakers the way forwards are. Goal suppression is more of a team effort and they can't really impact what goes on when they aren't on the ice sixty percent of the time, and offensively a lot of their points come from secondary assists and powerplay points. A D has to be like Top 10, Top 5 in the League to be a first overall pick in a vacuum worthy player (obviously if a Draft sucks then a good D is still the way to go).
Sabres have a 23 year old and 21 year old that both went first overall, and neither are busts or anything really close to it, and they still might not be a playoff team. That'd be pretty unfathomable if both were Forwards.