Why the Knox class was more capable than the Type 21

Was the Type 21 frigate better than the Knox class?

No; the Type 21 was a useful light stopgap, while the Knox class was a more capable purpose-designed ocean escort built in much larger numbers.

The Type 21 answered an immediate Royal Navy shortage through a commercial design that could be placed into production outside the usual process. The Knox class began from a more demanding requirement for a general-purpose American ocean escort. Building forty-six Knox-class ships also supported standardization and refinement on a scale the eight Type 21s could not match. The comparison is therefore less about national design quality than about the different requirements, budgets, and production programs behind the ships.