This must've differed a lot between usenet groups then; the ones I was on strictly followed a bottom (or infix) posting pattern and would chastise people who did it differently.
Technically we could be all writing in a giant Notepad file, and adhere to any discussion format.
(My experience is more than 10 years old, though)
In email, top-posting began when Microsoft’s first email client placed cursor and signature at the top of the quoted email instead, and didn’t provide commands to reflow partial quotes, or any of the features mentioned above. It also had no threaded view, which is what makes it practical to only partially quote instead of fully.
Culture is important, but technology can influence it heavily.