Ah. Yes, the diagram generating libraries are harder to implement. Since we don't actually implement the primitives that TeX uses, we can't just import libraries like xypic or tikz wholesale, we'd have to rewrite them from scratch. Maybe a project for when we've got most of the actual math working.
As for why we didn't work with MathJax, this started out as more of an internal project for making our own math fast, and has slowly built up to what we have today.
We also didn't think that we could get the speed we wanted while still supporting all of the obscure features MathJax does (like SVG rendering).
Support for tikz would be great, too.
More broadly: great project! Looks to bring us closer to the day when math on the web is easy and just works.
If you don't mind me asking, can you say why you didn't work with MathJax, and decided to start over?