It renders the maths server-side, so there's no runtime needed.
An additional bonus is that the resulting math is copy-pasteable, which in the case of disply math might not be that useful (since most equations are to complex to be meaningfully copy-pasted with unicode), but it helps from inline math dissappearing when copy pasting texts.
But, that being said, I'm sure they had their reasons to do so. For one, MathJax seems more well-known by quite a bit so maybe it's the safer option.
Too bad they're not using KaTeX [0] instead.
It renders the maths server-side, so there's no runtime needed.
An additional bonus is that the resulting math is copy-pasteable, which in the case of disply math might not be that useful (since most equations are to complex to be meaningfully copy-pasted with unicode), but it helps from inline math dissappearing when copy pasting texts.
But, that being said, I'm sure they had their reasons to do so. For one, MathJax seems more well-known by quite a bit so maybe it's the safer option.
1: https://katex.org/