Why do some racehorses run really well on turf tracks, but not on dirt tracks (and vice versa)?

Just a few things to add to the first two good posts. First, horses with thin soles that sting on the dirt will often prefer the gr(as long as it's not too hard and dry), and most will not run down on the turf, as they might on dirt. Big Brown is a perfect example. They've switched him to the gr because of his chronic bad feet and the fact that he was running down on all 4 legs. They don't even have to wrap him now, probably because his feet aren't stinging and the cracks are healing. Also (in the US at least), the reason horses seem to develop into gr horses when they get a little older and run better at longer distances, has more to do with the options available in gr racing than anything else. Turf courses are much more delicate than dirt tracks so only a fraction as many races are run on them, and very few works are allowed(only with special permission). Because of this, there just aren't very many cheaper races written for the gr - it is reserved for better horses and longer distances(which the betters put more $ on). So alot of cheap dirt horses that might actually be helped by the gr, never get the opportunity to try it just because they would have to risk a big jump in class. Also, the longer the race, the slower the pace and the less the surface gets chewed up. Another consideration is the fact that grass courses always have tighter turns because they are built inside of the dirt tracks, so the horses have to be able to handle that aspect. Did you know that in Europe, horses don't even train on the racecourses? They also run fewer races a day and the crowds walk the course and replace the divots afterwards, which I always thought was cool. They do all their training at home in rolling fields and plowed 'gallops'.