There are probably several things that are in play here.
Comparing ~230 *current* registration numbers to final attendance from last years is as 'apples to oranges' as it gets - can you tell me what numbers were in 2017 7 weeks before the event? I bet it was not 800.
Now - I won't be surprised if attendance is lower in the end:
1. It is much further away for CA players - what was a manageable drive in a car now became a flight, wich adds non-trivial cost. FL just does not have that many players to compensate.
2. For majority of players on the East Coast, I'd argue it's still a flight to FL, so not necessarily going to boost their attendance dramatically. For many players in the area JOOLA Teams is probably THE tournament to go to anyway, and these are only 3 weeks apart, so they are not going to rush to Orlando either (curious what's the historic participation overlap between Teams and US Open). JOOLA Teams attendance seems to be in line with previous years, by the way.
3. World Vets: plenty of folks from US attended it this year, so - many TT enthusiasts are now out of vacation time, disposable income, or both.
4. Orlando is not Vegas. This could swing both ways: probably a plus for families with kids, but other demographics might be happier in Vegas, just more stuff to do IMHO.
5. Format change: who knows whether it's a plus or a minus. Not being able to play in the Open might be a turnoff for some. New format did manage to confuse people at least initially, and it might be enough to suppress turnout. And by the way, figuring out whether new structure is a good thing or not simply from attendance numbers will be tough, since all other factors above are going to confound it.
6. Delay in registration start, payment processing issues - puzzled by that. It worked before, somehow...
We'll see how it works out.