There are a bunch of buttons it pushes in my lizard brain - the bright colours of the card art, the satisfying animations as they flip over and activate their abilities, the delightful use of haptic rumble, the nostalgia kick of recognising a lower-tier character from some comic storyline I read twenty years ago, or the link between characters and card abilities that makes each one memorable for that feeling of dread as the full scope of an opponent's scheme unfolds.
But the kicker, the thing that takes my phone battery down from fully-charged overnight to 12% by lunchtime, is the snappy (no pun intended), three-minute game length. Just long enough to feel like you can be tactical in your deckbuilding and placement strategy, but quick enough that both victory and defeat trigger that "ooh, one more can't hurt" impulse.
That it's light on the monetization and generous with its currency handouts helps massively, to the point that splurging on the season pass bundles feels like a pat-on-the-head reward I'm handing to the developers rather than a cheeky treat for my own "numbers go up" endorphin receptors.
I'm curious to see how it grows from here, though - they're already drafting some pretty obscure characters into the card pool, and I worry it can only ride this popularity wave for so long before it's deposed or a design blunder upsets the community.
Then again, people are still playing (and paying for) Hearthstone after more than eight years, so maybe the core is solid enough for long-term success.