The same reasons why we don’t have a universal spoken language in the first place. Languages were created a long time ago before international communication was easy. Sign languages were made in an era where you couldn’t easily communicate with people in different countries. So you couldn’t collaborate and work together on making a single language. Deaf people in America made a different sign language than the deaf people in England for example.
And convincing people to learn a different language and make that the new one they teach their children is very difficult.
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