Same reason there’s no universal language in general. People all around the world came up with ways to communicate independently of each other, there was no big meeting where they decided how to do it, and every time someone tries to force that to happen it doesn’t work because making everyone do things your way is a pipe dream at best and tyranny at worst. The development of sign language was *more* recent and *more* intentionally constructed than most spoken languages, but that effect is pretty hard to avoid anyway.
Because every timesomeone would try it didn’t replace the one people knew, it would just add another one to the options.
relevant: [https://xkcd.com/927/](https://xkcd.com/927/)
Also mute people might not be deaf and they would learn their natural language from listening to other people without sign language, taking over tose idosynchrasies, making learning a “universal” sign language instead of one that leans into their language even more difficult.
Also, see what happened to Esperanto, an artificial language meant to relace all other laguages when abroad. Didn’t catch on and the people who studied it kind of wasted their time.
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