It's a very simple matter of why he didn't take the shot when the car was on Houston - because he knew that if he missed, he'd have help further down on Elm Street. That way, if he missed, he would give himself more time to put the gun between the boxes, waltz down to the 2nd floor whistling a tune, buy an ice cold Coca Cola, crack it open and take a refreshing first sip.
He knew, too, that no one would suspect him because of his casual Coca Cola drinking demeanor. And sure enough that's what happened when he was confronted by Baker.