I do not know for certain, but this is my educated guess based on discussions I have read and participated in both here and on other sites, such as TA and TTOL. I think you are seeing the impact of three separate factors coming together (a sort of "perfect storm"), to cause a scarcity of units for non-owners, which will probably last for another year or two:
- A number of the timeshares on the island received significant damage during Hurricane Irma in September 2017 and required major rebuilding and in some cases almost total reconstruction. Some were completely shut down and only recently re-opened, causing an accumulation of weeks that owners could not use. Some have not re-opened. In some cases, the resorts are giving their owners priority to use those unused weeks first, before offering anything through timeshare exchanges, such as II or RCI.
- The pandemic hit just as the island was starting to finally get back to normal. The pandemic then resulted in the island being closed to visitors for several months, exacerbating the issue in number 1.
- The island is very popular in peak season (typically January to April) but also the rest of the year. Many owners use their weeks every year, so they never come up for exchange, which reduces the availability further.
I think you just have to keep your eyes open to see if something comes up. I suspect it may be a bit early yet for April/May 2022, but that is just a guess. With restrictions on travel in 2020 and 2021, there is also a pent-up demand for 2022. So you have a combination of a smaller-than-normal supply and a larger-than-normal demand.
I don't know if there are certain times of the year when units are released by particular resorts. Perhaps others with more specific experience will be able to provide that info. Because we tend to stay longer (3 to 5 weeks) when on the island, we tend to rent villas, rather than use our timeshare for exchanges.