Gooday All.
Sprint track racing requires maximum possible acceleration, tying and solidering of spokes makes two triangles and there for a much more rigid driving wheel than untied spokes. You also use high flange hubs to shorten the spoke length required. On a smooth Velodrome with singles (I think you call them sew ups) blown up to 150psi plus and really rigid wheels you can nearly fly - try it - diving off the banking is the best bike feeling ever.
For the road low flange hubs and long spokes untied makes a flexible wheel and much smoother ride.
David Rapley
Melbourne Australia.