Because people have different syntactic preferences? Because some people hate c-style brackets while others hate python-style white-space? While it would make life easier in the short-run for those trying to learn a language who have pre-existing knowledge of the standardized syntax, it's quite possible that some other syntax would lead to wildly improved programmer productivity or, at the very least, much more readable/aesthetically pleasing code.
In general, there seem to be a plethora of values which can be considered in choosing the syntactic form of a language: aesthetics, productivity, tradition, etc. Having different syntax between languages allows different niches for languages within these values not to mention allowing the possibility of great innovations which could improve the experience of programming. Seems unlikely that any current language has the optimal syntax with respect to maximizing all these values and hence it seems foolish to cut off innovation.