Why?
They have this encrypted parliamentary network and E-mail, so why do they need a messaging app?
I would think it's obvious. Why do the rest of us use messaging apps if we can use VPNs and E-mail? So they want to have encryption for another form of communication that the politicians want to use because it's convenient.
In the United States, the NSA had made a secure smartphone, but it was so awkward, hard to use, and out of date that they couldn't get Obama to use it. Why can't France have similar problems, even if they have their own agency with experts in cryptology?