I'd imagine they bought as many as possible several years ago, and only release 3-4 at a time. At this point, I doubt there are very many TLD's left that don't

I'd imagine they bought as many as possible several years ago, and only release 3-4 at a time. At this point, I doubt there are very many TLD's left that don't have a TPB domain, whether bought by TPB or by a reseller.