Opportunity cost. Both sports need an alternative to just running with the ball, because always using a single tactic would be too easy to defend against. In r

Opportunity cost. Both sports need an alternative to just running with the ball, because always using a single tactic would be too easy to defend against. In rugby, the only option is backwards passing, so that's what they do. In the NFL, forward passes are a lower risk alternative to backwards passes, so backwards passes are a much less attractive option.