My motivation for asking this question is that I was disappointed to see that the Python code in this post was in black & white.
Quoting former staff member @Oded:
We do not usually turn it on for sites that do not have a majority of posts with code in them (Lego, cooking, RPG are examples of such sites). Security tends to have more prose than code - hence, it isn't turned on.
Now, one could argue that MathJax is already ~1 MB or so (if I interpreted my browser tools correctly), which makes the 573 kB of Highlight.js less of a problem. (Yes, your browser caches those files; it's not 1.5 MB per page visit. Still, if you're on a slow 2G/3G connection ...)
It may be true that code snippets appear in a minority of questions and responses, but when they do, they can be quite valuable (e.g., for simulations or in support of various theory). Syntax highlighting can make such snippets more accessible, even to those not intimately familiar with the languages. Math benefits from various forms of popular expression. One that enjoys arguably broader use than math notation is code.