It was too early, but also lingered in development for far too long. It also never had the proper server-side implementations, so it could feel like a solution in search of a problem.

That said, the design is far more advanced than the rolling disaster which is Passkey and it is much better suited for small Yubikey-type devices, where you could easily have unlimited site support. It also had intrinsic portability and advanced real-world security considerations, such as an attestation that instructed a server to disable weaker authentication methods, such as email or SMS (which is also a customer support disaster, but still...).

Ultimately, SQRL is an object lesson that the best technical design doesn't always win - it needs the right timing and robust community/corporate support.

Edit: Also, the (client) reference implementation was written in x86 assembly language for Windows. So I'd say the timing, support, and portability are all reasons for the lack of adoption.