I bet this has a lot to do with it--there's way more competition between companies in the road and MTB wheel industry than in the track industry. Also, you're only taking into account factory built wheels--think about a custom built wheel, for example Road Dura-Ace with Open Pros vs. Track Dura-Ace with Open Pros. Dura-Ace 7800 road hubs aren't much heavier than Dura-Ace Track hubs, only a few grams. I think that there's much less of a market for pre-built pro-level track wheels than road wheels, because fewer people buy race-worthy track bikes complete.