Did you happen to notice what else was 'sold out' when you tried to book? Obviously, cabanas, but were there other 'high demand' things, as well?
Cruise by cruise, and depending on time of yesr, and repeat guests, things sell out differently. Best advice always do it ASAP when your booking window opens.
You maybe luckily, you may not, you may snag the last slot. Someone on my upcoming cruise recently snagged a palo brunch after it had been sold out a long time. They just kept an eye on the website.
Typically things go somewhere like this, but each cruise as said is different.
Castaway Cababas.
Palo brunch.
Remy brunch.
Nursery if required.
Port adventures with dolphins.
Boutique if offered.
Port adventures in Europe or Alaska to headline tourist places, Rome, St Petersburg, Helicopter rides.
Anything with horses, or golf.
Character meet and greets.
Character breakfast.
Palo dinner, Remy dinner.
Spa, formal days go fast.
The length of cruise and sea days gave an effect, the shorter, the less slots the harder.