The Type 21 made sense as a short-term numbers solution

Why were Type 21 frigates designed for conditions beyond those they could comfortably face?

They were a short-term, lighter frigate intended to maintain numbers and free larger ships until Type 22s arrived.

The Type 21 makes more sense when judged against the short-term force-structure problem it was intended to solve. Vosper designed it outside the usual specification as a light frigate to cover a shortage and release larger ships for other tasks. The class was less capable in demanding conditions than later ships, but that limitation was part of a deliberate compromise for numbers and availability. It was a stopgap intended to bridge the fleet until Type 22s arrived, without any expectation that a lighter design would match every capability of the larger ships.