Because the play had effectively ended when the illegal block (a stupid one too) took place so it was essentially a personal foul.
That was a little confusing....
Interestingly, an illegal block from behind used to be called clipping, but I haven’t heard that term used in years. Since the refs now announce the penalties over a mic, perhaps the terminology had to be dumbed down.
IBW (illegal block in the back) is a spot foul on a running play. The loose balll portion was done when the receiver caught the ball and “made a football move” Thus, the spot of enforcement is the spot of the foul in this case the +40. 10 yard penalty left the ball at the 50. The line to gain was around the -41. After consider the legally gained distance net the 10 yard penalty, the ball was left in advance of the line to gain which by rule is a first down. Had the IBW occurred while the passer had the ball, or while the ball were in the air, the spot of enforcement would have been the previous spot, and the foul would have been OPI assuming the pass crossed the line of scrimmage with a 15 yard penalty. (was not a screen pass). Were it a screen pass, the call would have been IBW but still enforced from the previous spot at the loose ball play is defined as all runs prior to the loose ball.
Simple.
It was not a safety because no part of the ball was in contact with the team in possession’s goal own line at the time the ball became legally dead.
It was a long yardage gain of 27 yards so the penalty brought the ball back 10 yards. Still a first down.....