Because it costs like $35 to buy the premium version on each release and people probably don't feel like spending that much money all the time.
I can't speak for anyone else, but for me personally I got tired of the MASSIVE amount of spam that developer sends. Like several messages per day spamming for this game and for "Rich and Youn". I eventually just unfollowed the dev and forget about the game. I just check back every few months and see if there's a new MEFF game because I like the twins.
