Decapitation strikes work well in highly personalist regimes (e.g. Russia). They work poorly in institutionalist regimes.

Iran is extremely institutionalist. You could keep killing leaders for years and they will just be replaced with similar ones.

That doesn’t work if a nation has strong institutions and hierarchies of command. Russia and Nazi Germany (and Iraq) were organized around a strong central leader who personally granted authority to his subordinates, but Iran’s rulers are given authority by a process. If the new supreme leader is killed, they will simply elect another one. Imagine that FDR was visiting Pearl Harbor when the Japanese attacked. Would the US government have collapsed? How many politicians and generals would the Japanese have had to kill before the US surrendered?

But that approach has demonstrably not worked in this instance because Iran has been planning for this exact scenario.