It has to do with fear of foreigners - which I imagine is highest amongst older french generations, where racism is probably more acceptable, and they have lived at least part of their life in a France that had very few non-European immigrants. And relatedly, it has to do with "law and order" and the fact that Sarkozy is seen as having a hard hand on these kind of issues, at least more so than Royal.

So its down to these two factors, I think.