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.

That would be wasteful - Signal is open source, works well, and has been audited more thoroughly than any proprietary alternative would ever be.

The real answer is: plausible deniability.