This^^^^
Well the preference would be to have DBs that are a step quicker and more athletic than the opponents receiver so they can always control them if I may expand on Bigbirds previous post.
When matched athleticism, the DB is already at a disadvantage due to having to react to WR moves. WR can fake looking for a catch and turn a DB. The DB is usually trying to REcover Ground as WR tries to separate.
The reason is more parity in athletic talent.