Defensive back isn’t always the position of choice for promising young football players. After all, at the college level, a starter will play hundreds of snaps per season, and top-notch players will record maybe five interceptions and 10 pass breakups. That means that they’ll make a highlight play on a very small percentage of snaps.
An elite defensive back will see even less action than that, as they’ll lock down receivers and force quarterbacks to throw elsewhere.
Getting outstanding play in the secondary is obviously extremely crucial to winning, but that lack of a spotlight has likely chased away some really good athletes from the position over the years.