I thought this was a bit strange too, especially as all the other courses (including Beset Of) are in the RGs at the minute. I was looking for an RG to play Cabo as I've been winning a few credits recently but apparently no-can-do.
My guess would be that they're trying to get more people to play Cabo before the release of the next TruGolf course. Perhaps because they'll lower/get rid of the green fees for Cabo and are trying to get people to pay while they can. I hope that's what's happening, anyway.
As the fees were just lowered, WGT could be trying to make people actually pay to play the actual course instead of just enter the RG, thus getting more credits in, but who knows?
Jack.