The technology supported and encouraged it, for example by editors placing the cursor and the signature at the bottom of the quoted post by default, and auto-removing the quoted post’s signature, so you could immediately start typing your reply at the “right” location. Furthermore, when reading postings, the viewer would automatically jump to the first nonquoted part.
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.
That's basically how discussions work in Wikipedia (and other MediaWiki projects). Discussion pages are just a giant text file which anyone can edit fully. The only extra feature over Notepad is an implicit edit history.
What made this workable was that discussions were rarely "hot" - typically you discussed with one or two users over a span of many hours / days. Once you got many people editing the discussion page at the same time, you got conflicts which were solvable but annoying.
Technically we could be all writing in a giant Notepad file, and adhere to any discussion format.