It is a common misconception (dare I say prejudice) that gestures are "universal" and therefore easy to learn/unify sign languages. Sign languages are a full-fledged natural language with nuances and complexities, just like spoken languages. Their grammar tends to be spatial as well as temporal, and a typical signer can express abstract concepts and its relationships using different placement/movement of signs. The reason why it's diverse is that most sign languages have been organically developed (just like spoken languages pre radio/TV era) in mostly isolated deaf communities.