it's not just for French. You just need to look at the US. When tipically Left policies get added to basic rights, people feel more safer - which is obviously good - but they also feel safer to vote to other sides: center, right, etc.

When basic rights get removed from the constitution, and those same rights become politicized (mostly the right wingers want to weaken them) then people get mobilized to vote against the right.

So in summary, it is in the interest of the Left to keep some rights outside of the constitution and keep them politicized.

Being downvoted.... I'm not saying I agree with it, just summarizing what is logical.