Given there have been literally dozens of remakes, reboots, reimaginings, reinspirations, rewhatever before us, I'd have to say there was a lot of luck involved. But this is probably the biggest factor:
Most projects immediately wanna make X-COM but better (can you blame them? have you seen the Suggestions forum? ).
This.
I was involved with UFO2000 a few years ago and to me the whole project went down after it was decided not to use the original files as default but instead make up all new sprites, weapons, etc., to prevent any future copyright issues. All of the sudden it stopped being XCOM and just turned into a multiplayer turn based game.
Plus, everyone had their own ideas of what would be just
awesome to add to the game: flamethrowers, multiple rocket launchers, Magnum 44s, etc and you had a ton of weaponsets that was very difficult to balance.
Truth is, most ideas being tossed into the Suggestions subforum are great for mods but
should not to be included in the build and should always be optional. Interface tweaks are usually fine but the moment you start adding new maps, weapons, units, missions, etc.,
you are moving away from the original game. And that causes more people to go rather than attract more people.
And I feel that even the current options available should be reduced, or at least, there should be a limit. Because when you get to a point where you have 100 options no one has time to try them all, and even if you try them all, you'll most likely find out that they aren't balanced against one another. Or, in other words, K.I.S.S. (Keep It Simple Stupid).