Because they play in Arena's which generally don't have a larger capacity than around 20,000. The nature of those sports limit the game day attendance. What a crazy attempt at a comparison of popularity. TV ratings, media coverage and annual revenues would show the big 4 are the NFL, MLB, NBA and NHL.

I'd place College Basketball and College Football ahead of soccer too....
That said, soccer has grown massively in the USA, mainly on the back of some huge investment. For sports to grow "organically" as we would need League to, schools and universities are where you need to start. Of the 450,000 registered players claimed by USA Union, 70,000 or so are in High Schools......and they have 900 College Teams.

Best chance for League is to get this new structure bedded in for a couple of years and then try to get the clubs involved in promoting the games to High Schools in their areas.

Pretty much agree with all of that

It is very important to build a strong regional base for the game, before starting to spread the wings throughout the country. Spinner Howland is a real dynamo for the game's development in the southeast. Hopefully the Northeast conference and the mid-Atlantic can emulate him. Intensification of current bases of activity should be the immediate priority.

However I hope that the heads of the USARL are looking forward to expansion. Chicago, Texas (Houston and Dallas), Denver, Phoenix, Salt Lake City (where there are numerous Polynesians), San Francisco, Los Angeles and perhaps Seattle should be the priorities if the game is to make the greatest possible leap up to a commercial TV contract. David Niu promised a national league with a national TV contract, but it turned out to be a false promise. That should not keep such a plan from being a medium to long term goal.

That's the dumbest comparison, what do you expect when comparing an outdoor code of football to an indoor sport?

What happened to the proposed Canadian Champs franchise btw?