Just a guess, but cruisers are heavier than destroyers and while carrying heavier weaponry they tend to be a bit slower than destroyers. DDEs were used as escorts during WWII as they could travel faster to the spot where submarines were sighted and unload their depth charges.
Perhaps my memory stems more from the days of WWII, but cruisers were big honkin' ships and destroyers were more sleek and streamlined. Things must have changed if you say they're now the same size!