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.
