In one word - money.
It is not just the ones that die on the race course, what about all the race horses/foals that are slaughtered for the meat trade? They don't all have a nice retirement in green fields.
http://horseracingshame.blogspot.com/2009/02/u k-racehorse-slaughter-foal-killing.html
This is one paragraph from the above:
"A few weeks ago, for example, the eight-year-old chestnut gelding, Cash King, which came sixth in a field of 16 at Aintree last May and won thousands of pounds in prize money, was rescued by an animal charity as he was about to be loaded onto a lorry for the slaughterhouse. A stable lad alerted the charity to the horse's plight"
Unlike pedigree dogs that don't make the grade for whatever reason the failed race horses are not very often sold to pet homes, the vast majority end up as meat, either for pet food here or for human food abroad. The ones that are slaughtered in this country are the lucky ones, at least they don't have to endure the travel abroad to be slaughtered in foreign lands.
Don't get me started on the double standards of the BBC!!!!!!
Especially as they are now using a show dog (St. Bernard) in Eastenders!!!!!