I wouldn't be totally shocked if the ST books were trade paperback-only when they come back in 2018 or 2019. The publishing industry has been moving away from mass market paperbacks for years now. Recently Stephen King's publisher confirmed that they were phasing out the mass market editions of many of his books. Trade paperbacks are a higher-margin format, so they're much more appealing to publishers. E-books have become the new mass market format.
Again, I don't know if this is likely, but it would not be a huge surprise, as it seems to be the way the industry is moving:
https://www.publishersweekly.com/pw...mass-market-dying-or-just-evolving-again.html
Apart from Star Trek all the new paperbacks I’ve got in the last couple of years have been trade.