The Buffalo Bills are loaded in the defensive secondary. Three out of their four starters (well, three out of five if you consider the nickel defense the “true base” defense) are legitimately among the top players at their position leaguewide. That’s not to knock the other two, as they are solid players in their own right; however, they aren’t at the level of Tre’Davious White, Jordan Poyer, and Micah Hyde.
In such a deep positional group, it’s difficult for younger players to claw their way onto a roster, as the veterans there are already established (and often have far more guaranteed money in their contracts).