The Socialists' vague appeal to Social Solidarity is a complicated code. To older workers it means the profession based system of occupational pensions and benefits, which though no great shakes, were there. But for younger immigrants, that social welfare system was not so well developed. A variety of newer projects were created to try to win loyalty to the state among those communities, but most experienced a more direct relation of repressive policing.
Sarkozy made that divide explicit early in his campaign to be the right's candidate. But once he had, he did not have to keep making the point, but rather could afford to sit back and let the left make it for him, by constantly warning that he was going to attack immigrants. Meanwhile, he was at liberty to make the argument to the more aspirational incomers that it was the Social Solidarity system that was holding them back - which has a grain of truth.
The Socialists were burdened with a largely negative campaign of scare-mongering ('Sarkozy will lead to riots' - i.e. you middle class voters ought to be scared of him provoking the migrants) because it was apparent that they accepted the basic argument that France's welfare state was holding the economy back.
On Doug's 'France lags' thesis, I am not sure that you can parcel up these discrete events into a general pattern. When France voted left, that was not so exceptional. There was a large welfarist Social Democratic movement across Europe. Greece, too, elected a state socialist government. Britain's Labour Party lurched to the 'left' in the late 70s, early 80s. And after 1982, Mitterand's government followed most Thatcherite policies, including the sale of 'titres participatifs' in public companies etc etc.