Is there a reason that different forms of sign-language were developed?

This. I once read a book about a sign language developed in an isolated village in the Palestine territories that had a large proportion of deaf people owing to a mutation. They had developed their own sign language and everyone in the village, hearing or deaf was fluent in it. Of course it wasn’t like ASL or any other sign language.

So the proliferation of sign languages is really just the same phenomenon as that of spoken languages.

Esperanto tried this for written/spoken language. Didn’t work.

Ne en unu tago elkreskis Kartago.

Stranger

The proliferation of sign languages (and new creoles in one generation) is very well covered in The Language Instinct by Steven Pinker, if anyone is interested. He makes a really strong argument that humans evolved to speak languages, and uses some of these examples as part of his argument.

Sed kiam kreskos kartago?