Square brackets for function arguments makes no sense, neither mathematically, nor programming language wise :)
Same for Sin requiring a capital letter, and curly braces instead of square brackets for matrices
Result: reserved key for brackets, instead of mixing brackets and numbers while making the only kind of bracket an entire key more to get to.
Evaluation: it's clean and efficient.
As for square brackets for parameters, it's unconventional, for sure, but not ambiguous because list literals use curly braces anyway.
Consistency and backwards-compatibility are much more important for most existing users, whatever their aestetic preferences may be.