They tried that in the '60s. The AFL-NFL merger kinda killed the idea because the players were getting paid enough to not need that one extra paycheck for the Loser Bowl.
Another (eventually) failed concept was the
Chicago College All-Star Game. The all-stars were elite college-level players playing against a given NFL team. They discontinued it after the Steelers were shutting out the college guys and it started raining near the end of the 3rd. The few fans in attendance left and never came back, and that was the end of that proud tradition. Doesn't help that the college kids hadn't won one in over a decade. Who wants to watch a foregone conclusion year after year?