Have never seen the NFL answer, but having seen various coaches answer this honestly, it's because:
A) Quick set up. It's long and hard to safely test everyone for maxiamal strength on various lifts. With the bench, you just set a bench up, grab a spotter and have at it.
B) A lot of older coaches, etc.... still believe in conversion charts. Basically they get the combine number A x B= estimated 1rm. On paper, it looks like you've got a big strong player. In real life, sometimes yes, sometimes no.
C) It looks good for the cameras.
A lot of strength coaches secretly opine that the C/P, C/J or even the incline press would be a better judge of strength because the patterns of motion more closely mimic what happens during plays(eg - tackles and such).