I think it has something to do with the fact that the game got competition.
Arena's can very much be compared with League of Legends in a way that both are arena type of games where positioning and how you manage your cooldowns are huge factors in victory, that aswell as team composition and stratergy.
To me, WoW Arena has been the most fun and intense gaming experience I've ever played, and to be frank, I don't even think League of Legends is as fun as arena was back in WotlK or TBC, but arenas and game balance has just become worse and worse over the times, and it has been casualized and alot less fun to play. So even tho I like the model of arenas much more than how league of legends work, I've chosen to play League of Legends now instead. And I know a TON of ex-multiglads / rank1's who has done the same.
Sure, arenas are awesome, but why play something the developers of the game doesn't give a rats ass about? Tbh if someone managed to release an arena game (or better yet) if wow managed to fix theirs, and make it competetive, i'd much prefer it to league of legends. But right now, the game just isn't as fun, and people are starting to see options, so we're no longer stuck with the less fun alternate.
edit: tired, middle of the night and i can't sleep, so its probably shit formulated, anyway as to why i lurk these forums when i'm not playing the game, is to occasionally check if it has changed anything towards the better on the arena side since i quit (which might make me wanna play again) but it doesnt seem like it