The APA is a business. They cater to players who average an APA 4-5 skill level. They do sponsor top players for events, such as exhibitions. I think Earl did some and Jeanette Lee and Fedor certainly have. But their main goal is to make a profit and evidently they don't see a way to do that with pro events.
I don't think it's the prize money that brings the APA players to Las Vegas. I think there are over 10k players now at the APA national events and the CSI combined events in March bring in about 13,000 bodies. On average, it's a moderately expensive vacation.
I think it would be great if US league pool did support a pro tour, but beyond CSI hosting the PBS events in March, I don't see it happening. If each league player in the US paid $5 per year towards a pro tour, that would be over $3 million. If a million came out for production costs of a tour, that still leaves two million for added money. It's not going to happen because that money would effectively be coming out of the pockets of the league businesses.