This is the best explanation I've seen. I think many commenters mean to say something similar but don't give enough context. Here's my phrasing of what I take away from tptacek's comment:

JWT is designed to be customized for a variety of use cases, which means programmers using JWT are rolling their own security scheme, including choosing cryptography from a practically unrestricted field of options. This is known to be a recipe for disaster. A good standard should provide an expert-validated scheme for a specific use case that gives end programmers assurance that if they comply with the standard, the scheme will fulfill its intended use case. This means standardizing separate use cases separately; therefore, JWT is at best a technology that experts could use (but probably wouldn't) to devise specific standard solutions for specific use cases which would then be appropriate for end programmers to use.