Because I have personally never seen "jazzes" pluralised and I didn't think of it.
Maths and math are both used, and the reason I used the plural form is not because I insist on anything but because it is the most commonly used of the two. I personally don't mind either forms.
With that said, linguistically using the plural for either is a bit odd, since that would imply you can pick "a" mathematic out of many, or "a" jazz out of many. But linguistic is not math (nor are lingustics maths), logic doesn't always apply.
Maths and math are both used, and the reason I used the plural form is not because I insist on anything but because it is the most commonly used of the two. I personally don't mind either forms.
With that said, linguistically using the plural for either is a bit odd, since that would imply you can pick "a" mathematic out of many, or "a" jazz out of many. But linguistic is not math (nor are lingustics maths), logic doesn't always apply.