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.
Even just a simple head movement depends on the culture, as nodding for "yes" and shaking the head for "no" is not universal [0][1].
And this doesn't even account for something like the "indian head bobble". [2]
Another example is the use of index finger and thumb to create an O. [3]
So if these very simple examples depend on the culture, I expect signing to be much more localized.
The challenge with having universal sign language, is that people communicate in over 300 different sign languages around the world.
Even in English speaking countries, like the United States vs. New Zealand, both countries have two different forms of sign language (ASL vs. NZSL) which are communicated amongst the Deaf and Hard of Hearing communities.
