Basically for the same reason we have multiple spoken languages. Sign languages were mostly developed independently by communities of deaf people around the wor

Basically for the same reason we have multiple spoken languages. Sign languages were mostly developed independently by communities of deaf people around the world, with occasional cross-pollination. People often think of e.g. ASL as a translation of American English, but that's not quite true – the grammar, idioms, puns etc. are quite different.