It's not so much secularisation as the general weakening of that issue across the board. Note that elsewhere in the country a lot of anticlerical bastions that voted solidly Left for over a century are now strongholds of the Right. Le Var rouge sounds like a joke now.
I don't think Var (and the rest of Côte d'Azur)'s rightward trend should mainly be ascribed to a change in issue salience. What changed first and foremost is the département itself - it went from a largely rural area of small landowners to a sprawly tourist-dominated abomination. And let's not forget about the pieds-noirs.
Pieds-noirs, like Mélenchon?
I'm pretty sure Mélenchon is the exception, politically speaking. So was Camus, for that matter. 
Melenchon and Camus were Pieds Noirs?! Learn something new every day.
Derrida as well, believe it or not.
