Why is there only one widely accepted syntax for mathematics?

There isn’t. About every specialized corner has its own conventions.

The derivative of a function, for example, can be written in at least 4 ways (https://en.wikipedia.org/wiki/Notation_for_differentiation)

Also note that https://en.wikipedia.org/wiki/Glossary_of_mathematical_symbo... lists only fairly common notation, but already says “may”, “in such-and-so field” or “it is also denoted as” lots of times, and mentions several notations with multiple meanings. The section on brackets, in particular, clearly shows the same notation can mean wildly different things.

There is some notation that’s almost universal, such as using ‘+’ for edition, but that’s the case for programming languages, too.

“It is a profoundly erroneous truism, repeated by all copy-books and by eminent people when they are making speeches, that we should cultivate the habit of thinking of what we are doing. The precise opposite is the case. Civilization advances by extending the number of important operations which we can perform without thinking about them. Operations of thought are like cavalry charges in a battle — they are strictly limited in number, they require fresh horses, and must only be made at decisive moments.”

― Alfred North Whitehead, An Introduction to Mathematics

Because it's very much a language if you want to formally communicate your thoughts. The 'syntax' becomes flexible when you're into higher levels of math. Feynman inventing Feynman diagrams comes to mind. This was one way math became more intuitive and flexible for communication of novel thoughts.