Earlier quoted context omitted.
Matrix is an open protocol and anyone's welcome to build a front-end, inspired by Telegram, that your mother would be happy using. Perhaps you meant the Element client?
Matrix is not a protocol, it's a product. Its openness is more like a public API to a product developed by one organization. Disclosure: I work with real open protocols.
Because they seem to like it and, based on my experience with the military being able to run it independently is a major consideration for them even for smaller systems.