I get the impression this is a tool made by technologists for technologists, rather than for people who spend a lot of time typesetting math. Academics (mathematicians, physicists, etc) don't have the same concerns as technologists, and the specifics of browser tech in particular are't relevant.

Latex is basically the mathematical (computer) lingua franca right now. Theres an entire cultural and technological ecosystem around latex thats been built up for many years. We even message each in latex notation when rendering isn't available! Theres decades of documentation online. Most important of all, its dead easy to use for most use cases!

Perhaps MathML is technically superior in some ways, but none that the people typesetting math are concerned with. That this is browser oriented tech in particular is already a disadvantage, as the browser is not a priority to us. In this way mathjax feels like the superior tool for users. It attempts to work with users rather than dictating to them.