It wasnt just that he was in his official residence, he was having a high level meeting with other senior members of the Iranian government. It seems likely the war started early (normally america strikes in the middle of the night) simply because all the people they wanted to assainate were all in one spot above ground and that was too good an opportunity to pass up.

Its really hard to imagine why anyone would do something that stupid when america just parked an aircraft carrier off your coast and looks to be spoiling for a fight.