This is about a decade old before MathJax and back when its predecessor, jsMath, was still pretty new. It was targeting students the most, not those who use latex professionally. I used it to create TidlyWiki notebooks for my students and it was something they actually did!

The goal was also about being translated into MathML. LaTeX is not necessarily concerned with mathematical sense while MathML (sometimes) is. I think this was also a motivation.

And, quite frankly, replacing \frac{a}{b} with a/b is a huge win for ease of writing basic math.