Why can’t Frankel go over to the the USA and prove himself?
Only one Triple Crown winner ever came to these shores – Omaha in 1936 as a 4-y-o who was runner up in the Ascot Gold Cup to the 1935 Oaks heroine Quashed (having won twice at Kempton in the predceding months).
As far as I am aware the last winner of a triple crown race to compete in Europe was 1965 Preakness Stakes winner Tom Rolfe who finished 7th to Sea-Bird II in the 1965 Arc.
Reigh Count famously won the 1928 Kentucky Derby and the 1929 Coronation Cup, and finished 2nd in the Ascot Gold Cup as well.
Derby-Preakness winner Carry Back ran in the 1962 Arc. but after a tough 4yo campaign and only a few weeks to acclimate he finished 10th.
But there isn’t as much incentive – in purses and potential stud value- to take an American horse to Europe as the other way around.
There have been several European horses to do equally well or better on the dirt; Exceller is the obvious example. Also Sakhee, Swain, Almutawakel, Arcangues, and Citation’s rival Noor (3rd in the Epsom Derby).