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.