Joined: Jan 2009
Posts: 44,127
Likes: 6,343
From: New York, NY, and High Falls, NY, USA
Bikes: 1962 Rudge Sports, 1971 Raleigh Super Course, 1971 Raleigh Pro Track, 1974 Raleigh International, 1975 Viscount Fixie, 1982 McLean, 1996 Lemond (Ti), 2002 Burley Zydeco tandem
Because that's the way it's always been done.
And that's quite an unsatisfying answer, isn't it? Now you got me curious. I don't really know.
Maybe it's because the screw-on nut may interfere if it is at the forward end of the screw?