It won because it was in place (Outlook) and almost nobody cares.
Official version is: "because there is a whole history of correspondence and it is convenient to forward it to new participants".
In reality? It doesn't matter. Almost no-one reads, neither top- or bottom-posted mails. But there is a drawback in top-posting and I mean a "my comments inside original post, in color/bold/with indent/randomly inserted between two phrases". There is no standard of citing in top-posting - thus sometimes original mail gone. Edited, re-edited and commented in various, inconsistent and often unreadable ways.
In my experience in corporate environments, that ability to forward to new participants with most of the context is really useful. If few people are going to read the history anyways, then in my opinion this edge case is valuable enough to tip in the scales.
I agree the difficulty quoting sucks, but that's mostly because of the switch from top posting to bottom posting. When people copy-and-paste the bit they are replying to and stay in the top posting paradigm things aren't so bad.
Official version is: "because there is a whole history of correspondence and it is convenient to forward it to new participants".
In reality? It doesn't matter. Almost no-one reads, neither top- or bottom-posted mails. But there is a drawback in top-posting and I mean a "my comments inside original post, in color/bold/with indent/randomly inserted between two phrases". There is no standard of citing in top-posting - thus sometimes original mail gone. Edited, re-edited and commented in various, inconsistent and often unreadable ways.