There realistcally is no reason to, its not really like there is any benefit.
The NFL is the highest revenue generating sports leauge in the world with tons of intrest and tv deals. It is by far the most popular sport and the sport that reccives the most funding in the nation with the largest economy in the world and the 3rd largest popualtion in the world.
Rugby wasnt even a pro sport till around the mid 90s. Rugby players make in the hundreads of thousands whereas most NFL players are mutli millonares. Rugby is playerd competitvly by about 10-12 nations most of whom play it as their 3rd or 4th choice sport. Even if you were to take the 9 countries that make up the six and tri nations they have a lower combined population than the USA