I also wondered about this, and imagine that the TB hoof isn't made for distance. It's like they bred TBs to race for three years and then give out. "willfull" and "reactive" are the traits I've seen too often in arabs, and why I've been trying to avoid them for sooo long. Funny!
So, if you ask me, the reason that TBs aren't out there "cleaning up" in endurance, it isn't because of their physiological shortcomings, it is because willful and reactive are mental traits that have been selected for in TBs, so most of them have it in spades, and THIS is what makes them unsuitable as endurance horses. We aren't all riding race bred TBs because we don't want to get killed :).
Laura "Not all who wander are lost." (jrr tolkien)
