Same reasons that you can't use many edge cases of email addresses (or even common "+"-addresses) with a lot of websites, even tough they are valid according to the spec:
- Most developers don't read spec and add e.g. validations in end-user applications where they think they are being clever
- Multiple fully fledged implementations of the core protocols are usually hard to come by. This means that rarely used features are that are not supported in all implementations won't be used and so organically wither
