I think that lack of official support from FFG is the biggest reason, and a (relatively) small and widely-dispersed player base is the second-biggest reason. There’s not the critical mass or top-down direction to get enough players onto an alternate format for it to be viable.
That being said, the Stimhack Online Cache Refresh tournaments are really fun and have succeeded in attracting a fairly hard core of probably 25 or so players that regularly play cache refresh games outside of the tournaments themselves (which seem to attract 40-50 players?).
As a group (and with dear leader @fightingwalloon’s fiat) we’ve decided on a post-FFG-sanctioned card pool for the next event and have actively discussed a few other variations to keep things fresh as time goes on. So we have approximated “stable and sufficiently large player base” and “top down control of the format,” fairly well and I think it will be sustainable past World’s 2018 for at least a while.