For the same reason we write colour in BE (British English) and color in AE (American English). There aren't any deep reasons behind it to be entirely honest.
It is argued though, that we say maths in BE because the word mathematics is a plural noun itself and therefore, it's abbreviated form should be plural as well. On the other hand though, we usually use the word mathematics as a singular noun. You say:
Mathematics is my passion.
Mathematics are my passions.
Therefore, it can be quite logical to use the word math as an abbreviation because its usage is in most of the cases, singular.
For me personally, I believe that it depends on wheter you want to speak British or American, I prefer the first one, so I say maths and write colour ;)