for some reason or another its more so the norm in racing to wear flat lens goggles. Don't know why, but it goes to show that different products appeal to different markets.
my friend that races told me one time that the flat lens tends to have a bigger peripheral because it can dip down further on your checks (bigger height from the bridge of the nose to the lowest point) without being awkwardly shaped. Look at the POC Retina goggle for a decent example