Because during the 70s, both the economic recession and deindustrialization caused a lot of those migrants to lose their jobs. Not to mention the large cultural gap between Europeans and non-Europeans. So in retrospect, it would’ve been much easier to simply repatriate them with payment rather than dealing with the issues associated with integration, poverty, and unemployment within those communities that still persist up until this day.
I find it "doubtful" that repatriating Millions of people would be "easier", and given the influx of former colonial peoples who couldn't go home, you'd still have issues.