Signal would probably be a poor base for an interoperability standard. Which flavour would you use? Signal Messenger, Matrix, OMEMO and allegedly WhatsApp all use the Signal protocol but can not interoperate at all.
Signal Protocol is also more complex than it needs to be. It has two levels of forward secrecy for example. It is basically all the crypto geekery of the last few decades packed into a instant messaging protocol. Something intended as an interconnection standard should be as simple as possible.