I've always wondered this about horse racing. You don't see this in any other sport. If a football player is injured, the media will report the extent of the injury.
In horse racing, however, because the athlete can't speak, his / her connections either say or don't say, but most of the time they don't say. Big Drama and Misremembered are two horses that come to mind. One had a long layoff, but ... I don't recall if anyone indicated what his injury was, and Big Drama hasn't run in ages - why? One of the few trainers I've heard state categorically the extent of one of his stars' injuries was Jim Cassidy (on TVG, I believe, and even the TVG reporter was shocked at how forthcoming he was).
I've had this discussion with a fellow TBCer and he thought it could have something to do with future breeding value, but I'm just not sure that's the most important reason. Anyone have any idea why all the mystery? Enquiring minds want to know.