I'll add to Meredith Peruzzi's answer. I remember once learning in college that there is a certain physical distance that a language can support. Once its speakers live beyond a certain physical distance, it becomes difficult to maintain the structure & vocabulary of that language. I.e. read more
For the same reason spoken language isn't universal. Sign languages develop organically, and although there is overlap between some (just as there is with Romance languages), each culture develops and expands the language according to their own needs. read more
That might seem logical but in reality it isn’t. Sign language developed independently in many places around the world and because the Deaf had no means to communicate (or sign) with their deaf counterparts in other countries, whatever signs developed locally could not easily be transmitted to another country. read more