Just spitballing here:

Iran has far more than twelve missiles. They could have hit a lot. Arguments that they "missed" are not particularly plausible, considering they managed to shut down a Saudi refinery second hand. Iran, for their part, is publicly playing up the bloodshed, while also saying "it isn't enough" while also pretty conclusively not blaming the US for either the Ukranian airliner nor the earthquake (because c'mon).

The phrase the war nerds of Twitter keep using is "escalate to de-escalate" as in "here, we fired missiles at you, now you can calm down so we don't have to go to war." Note that this is the first time Iranian conventional forces have struck the United States, other than shooting down a drone (that was most probably in Iranian airspace). It's really not their style.

It would break with centuries of precedent for Iran to get all troops-in-field over this one. Symmetrical response, from their point of view, would be murdering Giuliani. I don't think Iran is done, but I also don't think this was Iran starting. This, from my read, is Iran going "here's that response you were looking for, now settle in because by the time we've had our say you'll have forgotten how pissed we are and your people will have moved on."

South asia does vengeance with remarkable patience. If Iran wanted to pop off a few missiles for the sake of popping off a few missiles they could have done it within a half hour of Suleimani being assassinated. This is something else and everyone pretending otherwise is eating the conventional narrative with a spoon.