TGJB

>And VG is one of very few horses of the last 10 years to run in all 3 TC races and go on to have any real success at 4.<

I think rate of development has something to do with this. Early bloomers would tend to do better in the Derby than late bloomers because it's so early in the year. The typical Derby winner is not fast enough to beat the best older horses. Not running well at 4 could be a matter of their peak early bloomer form at 3 not being good enough to do well against the typical faster 4 year old. I think we see a lot of that when horses go from 2 to 3 also.

Some horses have been retired for economic reasons.

However, there's little doubt that the Derby preparation and the triple crown are grueling. It's certainly no shock when some of those horses wind up with physical problems later in the year.