My guess is: the US can’t directly blockade the straight’s choke-point or they would be in range of Iranian anti-ship missiles, speed boats, and other defenses. So they are stuck having to blockade a multi-thousand km long band of ocean 100s if not 1000s of kms radius away from the actual straight. Even with all the boats they have there, that’s just too big, there would unavoidably be gaps that boats could pass through before the navy could react.