Why extra Allied submarines had uneven value

Would extra American fleet submarines have materially helped the smaller Allied navies?

Only in suitable theaters; submarines are strong sea-denial weapons but weak substitutes for sea-control forces.

Simply handing more large submarines to small Allied navies is not automatically transformative. Their value was greatest in constrained Mediterranean waters and in the Pacific campaign against Japanese shipping. The same limit applies to a hypothetical American Type XXI-equivalent: better submerged performance helps where the Allies need sea denial, but it does not perform escort, patrol, or amphibious sea-control work. Elsewhere, Allied navies more often needed forces that could exercise sea control.