I think the difference is, in the 1940's, pro football wasn't a big deal yet. College football was more popular, and of course baseball was absolute king in this country. Also, part of the Rams moving from Cleveland to Los Angeles was a stipulation that the team become integrated, so signing black players was forced upon them. That's a little different than Branch Rickey going out on his own to sign a black player (against the unwritten rules), and baseball's popularity made it a much bigger deal than in the NFL. Plus, Robinson had a 10-year careeer and became a Hall of Fame player and hero. Washington had three ok years and was done.
Interesting note, Washington and Robinson played football together at UCLA, both in the backfield.