I'm actually curious as to what triggered this latest digital card game fad. As far as I can remember they've been around for over a decade, but have never been very popular. Magic the Gathering online game came out in 2002 but was never a huge hit.
I know blizzard probably mainstreamed it, but blizzard has never been original or innovative, so there must have been some other popular digital card game(s) before them that gave them the idea.
Developers figured out they were easy and cheap to make (Hearthstone was made in Unity) and had that pre-built-in "booster pack" economy that could fuel microtransactions. That's what it boils down to. That's why these card games (or card-inspired) games are so popular with devs now. Bethesda has Elder Scrolls Legends, king.com has Clash Royale or whatever, and so on. That style of game is really easy to add a "gambling" mechanic to where you pay money for random shit.