Because now the surprise is blown, and Hezbollah no longer has any forward presence in Lebanon. Iran is crazy, but not stupid.
I seriously doubt that Iran predicted the IDF response to the relatively minor provocations Hezbollah provided.
I don't think Iran expected Israels response either. But then again, I don't think they were looking for that.
I think they were looking for the UN to get a Lebanese force in place to assist the Hezzies with resupply and a buffer against Israel. Now, if Israel goes back into Lebanon, they will have to deal with the Lebanese army who will say that they are being attacked by Israel and this will be Iran's and Syrias excuse to join a war.
There is a lot going on here.