DBs can play into their 30s, and it's harder to find them in the lower rounds of the draft. Whereas impact running backs can even sometimes be found outside the top 100 of a given draft, and definitely outside the first and second rounds.
Plus, Ramsey hasn't been suspended yet. He was a possible fit at both CB and S going into a season where we had Carr, Claiborne, Church, and Wilcox all leaving in FA the following season. If we didn't hit lightening in a bottle with Anthony Brown, who knows where we'd have been in the defensive backfield in 2016. It ended up costing us in the playoffs, anyway.