From the NTRA interview with Barbaro's vet:
Media: Dr. Richardson, just out of curiosity. In our quest for speed in horse racing are we breeding a less sturdy horse?
Dean Richardson: Categorically no. No. I think really when you - really, look at thoroughbred race horses. I mean, the incidence of injury in thoroughbred in racing is not any higher than it ever was. It's still a risky endeavor. Horses are going fast. These are massive animals running fast. There's no evidence whatsoever that the prevalence of injury in thoroughbred racing is increasing.
Horses used to be raced far more frequently than today because the purses just weren't as good and you couldn't make any money unless you raced every week. Today, you can win fewer races and do fine, so you can afford to lay them off more.
When horses were raced every week in the past, the ones that were going to break down did so long before the BIG races.