Because Europe would not use this to properly bring back actual pure diaspora. The moment some half-kenyan/half-(euro diaspora) gets rejected, it would cause controversy for the average milquetoast modern European civnat "far-right" movement

On the other hand I could imagine (and want) glorious Japan to do something with the millions of pure Japs trapped in the Americas (especially Peru and Brazil) because I trust the Japs will be based enough to restrict it to pure blood