It's a chicken egg problem. Developers use JWTs because it's what they think they know. Companies build libraries to support what developers are using. Security engineers say JWTs are easy to screw up [1]. Newer frameworks offer ways to move off of JWTs. New programming language comes out. New frameworks built for that programming language. What is someone most likely to build first as an integration? What developers are using. JWTs become defacto for a new framework. Security engineers report the same bugs they've seen. Even more languages and frameworks come out. Rinse. Lather. Repeat. Write up the same OAuth bug for the 15th time.

[1] http://cryto.net/~joepie91/blog/2016/06/19/stop-using-jwt-fo...

Edit: I was actually writing this code tonight myself for a project instead of it already being baked into the platform framework because SSO is only available as an "enterprise" feature and it's $150 a month for static shared password authentication. So market forces incentivize diverging standards.