Neither does DuckDuckGo. Been a feature request for >5 years now.
I'm 99% certain that they turn off IPv6 to avoid complaints. Implementing IPv6 on a frontend load balancer is a trivial networking change. But the only way to ensure an IPv6 connection works is for the user's OS, networking, firewall, router, modem, ISP backend network, ISP DNS resolver, target website DNS, and target website load balancer & firewall, all have IPv6 configured properly. If a single step is misconfigured, or uses IPv6 tunneling/translation, every request might be blocked until the website disables IPv6. So don't support IPv6 at all and you avoid headaches.
Avoiding an unnecessary support headache is the basic reason why IPv6 has existed for 26 years and yet Google can still barely get 40% use for its own website. Everybody loves to design a spaceship, but nobody designs for moving between spaceships mid-launch.
