Because they were largely US funded.
That’s why they have USAF serials….64-18265.

I recently saw 64-18264 at Pima…

There was German money in there too and the aircraft were supposed to be divided up equally between the three nations (USA, UK and Germany) at the end of the trials. But, by the time the evaluation started, the Germans had lost interest and only sent a token group of six personnel to participate. At the end of the trials, they were uninterested in the airframes, but were definitely interested in the Pegasus engine, so they did a trade with the Americans to swop airframes for engines. By this time, we were down to 8 Kestrels, having lost XS696 early on. The UK, for whatever reason, took the hit on the lost aircraft, so only took two, leaving the USA with its three plus the German share, totaling six in all.
My ambition is to get over to Pima one of these days and have a photograph taken with XS 690. It has been mostly restored, except for the nose cone, which got lost somewhere, so the museum substituted a Harrier nose cone. It doesn’t look the same as the original, but better than nothing.