MathJax 3's synchronous approach requires you to load everything you need up front — all the font data, all the extensions, etc.

That makes for larger payloads, and and doesn't allow dynamic extensions (like \require{...}).

But there is only one file needed, and it can be packaged as part of a larger file containing other code needed by the page.

This means every combination of components needs its own custom packaging.

That makes distribution via a CDN mode difficult.