Mostly that the coffee shops are the customer of the captive portal solution and the shop runners don't know, don't care, some combination of both that the coffee shop users have IPv6 right now so there is no incentive for the captive portal software or vendor to push the matter. Typically at this point "allowing IPv6" also really means "running two versions of the captive portal and intercept stack" rather than "moving from IPv4 to IPv6".

You get what you pay for. Non-revenue generating services are setup and maintained as cheaply as possible. They're not going to buy extra IT labor to test IPv6. In fact they go in the opposite direction by locking down ports to just ports 53, 80, and 443. Other ports are blocked because non-web traffic includes bandwidth hogs like BitTorrent & Zoom, spam over port 25, and copyright infringement letter generating activities. The most annoying networks even force DNS and HTTP through a proxy leaving TCP 443 as the sole pinhole to the Internet.