They have internal highly secure chat systems with locked down hardware. They probably prefer Signal as it is not logging everything they say for internal review and they can chat with friends, business people, lobbyists. Just guessing.
My theory is based on my own experience of running my own locked down chat systems. Most people will prefer Discord but will use my systems in the rare cases Discord goes offline for a prolonged period of time.
Government officials are required to record their communications. Signal is also not approved for secure communications. And while Signal the protocol and company might be secure, the phones it runs are not.
