API

One of the success factors for regular TCP and programs using that, is the standardized socket API. It has well defined functionality and using this API you can move programs between many different operating systems as TCP works the same.

QUIC is not there. There is no standard API for QUIC.

With QUIC, you need to pick one of the existing library implementations and stick with its API. It makes applications "locked in" to a single library to some extent. Changing to another library means another API and that might involve a lot of work.

Also, since QUIC is typically implemented in user-space, it can't easily just extend the socket API or appear similar to how existing TCP and UDP functionality do. Using QUIC will mean using another API than the socket API.