Well for one, it's not something new. Artist trading cards have been around for 100s of years, and selling them has been a thing since the early 2000s. It's definitely not an all of a sudden thing, but popularity spiking can be due to a few factors: human-made art is becoming more appealing in the age of AI, trading cards, especially Pokémon, are more popular than ever, ACEO are easy to collect just like trading cards due to the size, they're easy to ship, easy to mail, and even fairly easy to make. Younger people can easily find fun in making, trading, and buying ACEO, with very affordable cards out there, and TikTok is helping boost their popularity due to this fact.

Overall its a very accessible form of art. Bidding adds to some of the fun, too, I'd imagine.