Think about the physical aspect of the standing start. The straight arms, out over the pedals and tight body to get all the power down to the pedals. Then think about the road bike without any bar mods. You need your hands forward in some way to change the gears. They either have to be forward in the drops to reach the shifters and you have to change the grip to shift, or they're on the hoods where your upper body is high. I just don't think the body position allows maximal effort like a track bike does.