1) No pre-reqs
Pre-reqs are one of the main reasons why some games don't have boots while others have many booted - with good pre-reqs, many of the unreliable players are unable to join. The Double Earth FFA does not have any pre-reqs.
2) An Auto-Game
Auto games, in general, tend to see more boots. These are the games that newbies will flock to, as they're open to anybody. Some of these newbies may not stay, and as the Double Earth FFA is a multi-day game, they will likely be in quite a few when they forget about the site/quit.
3) Veterans DON'T play the setup
Double Earth, is, honestly, not a strategic template. It's value decreases a lot after you gain a few levels and are able to make better games. First of all, it's FFA, only 4 players, and has light fog. It's not big enough for a diplomacy or a large FFA. It's not for people who want a strategic template. There are MUCH better templates out there, and people can easily make those games.*
*While you hear people complaining about Strategic 1v1, it's a playable template from beginner to professional. There are good people who still play Strat 1v1 auto-games. Honestly, in my opinion, Medium Earth's strategic 1v1 is the best compromise between being beginner-friendly but still having a good top-level game.**
** This is why I support Medium Earth over other great strategic templates. Those templates may have better top-level games, but they don't really seem beginner-friendly. (Roma, EA&O, Chaos' Maps, Europe, Gui's Maps, 0% SR)