One issue with QUIC in e.g. C is how heavyweight it feels to use compared to garden-variety BSD sockets (and that’s already not the most ergonomic of APIs). I haven’t encountered a QUIC library that didn’t feel like it would absolutely dominate a simple application in both code size and API-design pressure. Of course, for large applications that’s less relevant, but the result is that switching to QUIC gets perceived as a Big Deal, a step for when you’re doing Serious Stuff. That’s not ideal.
I’d love to just play with QUIC a bit because it’s pretty neat, but I always get distracted by this problem and end up reading the RFCs, which so far I haven’t had the patience to get through.