Because touch screens are very different from typewriters and having to precisely press tiny keys without making full use of their capabilities is extremely inefficient.
The two-tier reply system on SO is really useful and would be harder to implement -- the replies to the questions, but also replies to the posts/replies. I don't know how that would look if starting from Lemmy as a base.
How so? Lemmy allows unlimited nesting of replies, which is even better.
https://www.youtube.com/watch?v=Lh8z9fFJgVA