The fewer people who you're required to trust, the fewer breaches there will be, even if no one is malicious and those breaches are accidental.

And ultimately even if the people you are talking to and your messaging provider are 100% trustworthy and never make mistakes, they usually cannot resist a lawful government request for data. Signal just has virtual no data available to provide them; Telegram could give them entire chat histories, and could be required to provide access to in-progress chats.