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.
