I think a lot of the reason is that, ok, LaTeX is extremely complicated. We all know this. Its partisans tend to believe that this is because typesetting is a hard problem. Typesetting is a hard problem! We know this, too. But I think that a substantial fraction of the complexity of LaTeX is accidental complexity stemming, ultimately, from the inherently loosey-goosey nature of the system.
See, there is no real abstraction in TeX. There are no real mechanisms for encapsulation/information hiding/whatever. It's all just characters that eat characters and turn into other characters. Anything can do anything. Anything can be anything. As a result, the whole "theory" of what a TeX program "is" (like in the sense that Peter Naur used that word?) is conventional.
This means that even to reuse other people's code, you have to imbibe decades of convention that's maybe semi-documented, maybe folkloric, or maybe perfectly sound but requires you to read a 230-page manual. I can only speak for myself, but for me, it's no way to live.
Now, again, TeX-lovers tend to claim that its high degree of loosey-gooseyness is necessary, that it's what makes TeX "powerful." I don't know, that sounds to me like the same old "you can't handle writing assembly" story. Sure I can. I'd just rather not if I can help it.
Maybe it's the PL person in me.
```
pandoc input.html -t typst -o output.typ
typst compile output.typ output.pdf
```