BigBlue,
Check this out
http://www.derbytrail.com/forums/showthread.php?t=39844
and click on the link to the article (pdf) that Steve put in the initial post. There are answers from people "in the know." They all have some merit, not necessarily applicable to all instances.
There are a lot of horses, mostly older geldings in lower lever races, who race with much greater frequency. Are they more durable, tried and true? Or is it simple economics?
Lots of answers, but no simple one that can be applied across the board.