Mostly because track hubs were never quick release and it doesn't make sense to modify them that way. Couple this with the rumors that quick releases were not strong enough for fixed riding and the fact that a quick release makes it harder to set chain tension without tugs and you get a lack of them on road fixed gear bikes.
Road fixed riders that ride often; don't change cogs that much and hardly ever flip their wheels.
Here's what Sheldon has to say:
Track hubs generally come with solid axles and track nuts. This is because most velodromes have rules requiring this type of wheel attachment.
As a result, it is widely believed that quick-release axles are not suitable for fixed-gear use. This is false!
It is my belief that the velodrome rule dates back to long before the invention of the quick-release, when the choices were standard nuts or wing nuts. The protruding "wings" of wing nuts might constitute a hazard in a crash, and I believe that's why the rule was instituted. Since quick-release skewers don't have any sharp projections, that becomes a non-issue in practice.
If you are going to use a quick release with a fixed gear, you should make sure to use a good quality enclosed-cam skewer.