Indeed? Big Question is WHY? Why not a D520 or even a Hawk 75? Perhaps that it used construction materials that were less scarce? Meanwhile, French factories were building German aircraft.Dornier built them?
The P-51 didn't exist when Dornier was manufacturing the D-3801.So someone was fool enough to keep buying them. Even in an era when cheap P-51's were available.
They did buy P51s for the interim until their licenced home production of Vampires was in action, but the Moranes were a quick fix in the meantime. The rest of the Moranes and Me109s were well worn out from years of neutrality patrols. By 1947 the Swiss had tried out and decided to standardise on DeHavilland Vampires which were delivered. See Swiss MustangsSo someone was fool enough to keep buying them. Even in an era when cheap P-51's were available.