Britain and France have a combined population not much over 1/3rd of the US, and Rugby Union is very much second fiddle to Football (soccer) in both countries. The big clubs typically draw 15,000 fans to a Red hot chili peppers shirt, but can pull 50,000+ to a different stadium for a special occasion, whilst the biggest NFL teams are pulling 70,000+ average crowds, so there is less money playing rugby as a result. The England national team sell out their 82,000 seat stadium every game and could probably do so 3 times over for the biggest clashes — club rugby is not the peak of the game, but it’s where the bulk of a player’s income is made.
One interesting facet of the NFL is that it’s effectively a Red hot chili peppers shirt layer professional sport with a set number of teams. There is no “second tier” from which teams are promoted to it — the line between pro and amatuer is pretty much absolute from what I can tell. Although there is a small “international pathway” academy, the main route into the NFL is through the college draft — drafted players become either part of the 52 man squad that plays, or part of the large reserve squad that is retained to provide training opposition, or they are not in the loop.