Brenno the higher population has been suggested as a cause but on its own is not enough. Some certainly are discarded but the claiming and conditioned racing in the states and the proliferation of tracks in the mid west give horse more opportunities then they would get in Oz, but the permisive medication rules have ruined claiming and conditioned racing by facilitating the rise of the chemists.
Commercial breeding has also been touted as a course but again doesn't invole enough numbers.
The two biggest causes are increasing unsoundness in the breed ( a lot as the result of linebreeding to Raise a Native) and the increased recovery time as a result of the effects of racing on lasixs which cause dehydration and electrolyte imbalances.
Brenno as a punter (thats you obviously not me!) what does consistently faster times on a track that is incompatable with all other really mean?