As the local, it's my duty to answer the question. And the answer is simple : nobody trusts any politician here, & noone votes.
I terms of numbers FN does not progress since 20 years. Yet, as less & less people go to vote for other parties, the voter's share of the FN increases.
Chirac did nothing relevant. Sarkozy made a lot of things, and then went backwards. Hollande is turing around real problems, taking care only of secondary problems(as school's schedules or the gay wedding). So, both conservatives & leftists are not motivated to go voting.
The nationalists, OTOH, are very motivated. Their racist hate speech is more & more heard. Plus, the moral left just says to them "you are stupid, wrong & evil", which motivates them even more(even if true).
As other countries, France did see a lot of commoners be victims of globalization. They need someone to be guilty of their own demise - and the arabs perfectly fit. Blacks are just seen as inferiors by many, but not cared about a lot. Arabs are hated throughout the country by everyone who has seen its lifestyle threatened last 30 years - and that's a lot of people.
Most motivated wins. FN voters are more motivated than others, because others are tired of the failure of their champions.(wait, that's not even failures, they did not even try).
Situation's dirty here. As a reaction to racism, more & more people turn towards religion - mainly islam, but american evangelists take their toll, too. The country is divided, losing its economical power, its health efficiency, its school system. In other words, France is in decline.
It happens to all countries, sooner or later. It's rather sour to experience.