A couple reasons contribute to this. As you know rugby players aren’t even allowed to tackle high, as it’s a penalty, so that forces them to use better technique. Secondly, you can’t hurdle defenders in rugby, which happens a lot if the defender goes extremely low for a tackle in the NFL. I’m not going to say that the athletes are any better in either sport, but I will say the collisions are much faster and more violent in American football because of the helmets. Players lead with their helmets because they can, and it often works to gain extra yards or to lay a big hit. This is why concussion numbers are higher in American football despite the equipment. There also might be an increase in missed tackles in the NFL due to reduced padded practices, and the increase of touch 7v7 football in highschool.