This is like asking why there isn't one universal language for human speech. Both are due to long storied histories involving many different groups of people, t

This is like asking why there isn't one universal language for human speech. Both are due to long storied histories involving many different groups of people, trying to do very different things, and achieving something which meets their needs and is becomes very established. Having One True Language/Syntax may be interesting to think about, and could theoretically make things much more efficient, but you'd never be able to get people to agree on things (we can't even agree on the "proper" way to do braces), the transition would be an impossible nightmare, and the end result could be something which is good for many tasks but excellent at none.