Difference in culture perhaps? Americans, historically at least, don't recognize titles from kings and queens to knights and ladies. There isn't any reason they should dress for the occasion as there is no special occasion. Oh, OK, they of course, realize that he's special in the UK, but I doubt concepts of protocol make sense here. After all, that is one of the things we fought for in the Revolution (and we do call it the Revolution rather than the War of Independence).

Greenville may be further west in S.Carolina than most battle sites, but they do say more battles were fought in that area than in all the other states combined. That was long ago, but I suspect the Carolinians are taught their history.

As for how he's dressed, I think it was a perfect and practical choice. If he looks a bit tropical, well he's pretty close to the tropics!