My guess: Kupp was the first read on this play. Goff thought he had him and therefore did not even look at any other option.
(That's my understanding of a QB going thru his progressions: you look at option 1. If you can get him the ball then do it. If he's covered then and only then do you look at option 2. If '2' is open throw him the ball; if not then look at 3, etc. So it really doesn't matter if 5 guys are running free in the end zone: if #1 is open (or, more accurately, if Goff
thinks #1 is open) that's where the ball is going, even if other guys are 'more open'.)
AlbaNY_Ram