Probably because you're trying to break up your changes. Two good blog posts that talk about this workflow are:

https://jg.gg/2018/09/29/stacked-diffs-versus-pull-requests/

and

https://kurtisnusbaum.medium.com/stacked-diffs-keeping-phabr...

Those were useful, thanks!

There are two features you want to add, called "comments" and "reactions". Those aren't PR lingo, they could be named "Feature X" and "Feature Y".

Since I don't use git much these days, I'm going to speak in generalities.

I would guess the assumption is that you can't have reactions without comments. So comments v1 is "comments" and v2 is "reactions".

They need to be different PRs, so I guess that's what leads to different branches. But they can be reviewed independently, you just have to merge one before the other.