Southern Californians are notoriously fickle sports attendees. Too many other entertainment and outdoor options.
Pennsylvania, Cleveland/Columbus, Iowa City, maybe NJ could have drawn decent crowds. About 1500 just watched two solitary Final X series in a high school gym in Columbus. The star power on the US U23 roster and the novelty factor of a world-level event would have drawn elsewhere.
Vegas was never going to draw any fans for U23. Way too under the radar in an entertainment-bloated town. Not in a sports-watching, wrestling-watching hotbed region.
Worlds next year should be better because more diehards will be motivated to go make a vacation of it. But it still won't draw like it could if it were near people who more readily attend wrestling events.