I loved the idea of Hearthstone and still think it can be a supremely fun game ... but someone said "it's about chasing the meta" and that is so true.
The minute they started releasing big card-packs/expansions in fairly rapid succession, I stopped playing and following the game. It quickly became a "get the latest cards" and "find the most OP deck and copy it". We're now, what, 4 or 5 expansions in? There are so many cards out there, their attempt to "exclude" certain packs and make Standard vs. Wild games isn't helping. For me.
TL;DR -- too difficult for new players to get into with the massive pile of cards/expansions, too difficult for old-timers to get *back* into for the same reason.