I don't think it's entirely true. Many have managed to create alternatives that have gotten serious traction. There's a few I've been consistently playing for a week and honestly enjoying more than Wordle itself.
I've been playing Quordle, and sometimes it's more insane alternatives [1] [2]. I've also really been enjoying Semantle [3], it's probably one of my favorite and most unique ones. There's other ones that are also need but maybe too much, like Squabble [4] and Squardle [5], but I wouldn't say they've all failed.
Also, to say "the game itself is mediocre at best" seems ridiculous. A mediocre game doesn't get popular purely based on once a day and sharing...