I don't understand why you think 20 teams is not enough? At every Alter Cup I've been to, there were under 10 teams that really stood out from the rest of the fleet as the contenders.
This is supposed to be championship representing the top sailors from the US multihull community. 20 teams is more than plenty. Quantity \= quality.

I think your problem is that no one wants to join US Sailing unless... well no one wants to join US Sailing I suppose until people get enough in return of what they deem to be worth the expense. For everyone that is going to be different.

For me personally I would pay a much larger membership fee to a national organization that represents multihull sailing, has a development program, and a great championship.

Not only does US Sailing offer none of those for me, my personal experiences dealing with them as a multihull sailor (Miami OCR, Olympic trials and the World Sailing games) has been nothing but a complete disappointment.

But hey, I know the sailors aren't important. You just want them to join.