They’re two polar opposite 5* courses. Pau is one of the flattest 5*'s on the calendar and getting the time is about rideability and efficiency through the combinations, it’s kind of a sprinter’s course. Maryland is one of the hilliest and getting the time is about having a relentless gallop and a bold jumper, it’s kind of a distance runner’s course. Off the Record is a great example of a horse that’s well suited to Pau, but probably not so much Maryland.
Also, most of these riders aren’t footing the travel bill, it may just be that their owners want to have a nice French vacation in the fall. Can’t say I blame them, Elkton, MD is beautiful country but do a quick google image search of both locations and I think you can see the appeal of Pau from an owner’s perspective .