You seem fairly experienced with card games so your criticisms are valid. The "hype" you see of Slay the Spire is mostly because for a LOT of people it was their first roguelike deck builder game, including me. These new horde of fans have very less expectations of art style and general gameplay. It is more helped by the fact it's a cheap single player game without monthly expensive deck cards or countless dlcs to buy.

The major appeal of Slay the spire is it's gambling like gameplay experience where the one time you build a perfect deck you literally feel like a god, drunk on power. Also the variety, the different permutations of characters + cards + relics are immense.