Maybe a weird design decision but it never bothers me. I don't use search a lot, but when I do I just press End on a desktop browser to immediately scroll to it, or on mobile I fling the page all the way down to the bottom. Doesn't bother me, personally.

I also kind of appreciate the fact it doesn't take up space above, so the search field is a little bit wider than what is normally available in other sites where it is on top.

I'd guess because there it is easily accessible, but out of the way since it's likely one of the lesser-used features.

I didn't even notice/forgot it was there!

I usually just went to hn.algolia.com manually

Bottom of page never bothered me, but I wish the default sort was by date, since the relevance ranking is almost useless and I almost always want the most recent hits first.