Because they cause more problems than they solve.
A lot of the implementations _aren't even faster_, but the laundry list of security bugs they have caused is very real. Suddenly you get to worry about irrevocable tokens in order to solve scaling problems you don't have.
A good intro is: http://cryto.net/~joepie91/blog/attachments/jwt-flowchart.pn...
Can you point to a RTFM on why not? I'm sure it's a big list of reasons but where can I read about the biggest one?