Signal has always aimed to ensure confidentiality in the simplest way possible. People forget that there are anonymous systems or systems that do not require a telephone number but they are incredibly painful to set up. You either have to go through physical checks with QR code exchanges to validate participants or have some kind of web of trust (no one has fond memories of PGP key signing parties).
The same goes for decentralization. On paper, everyone wants decentralization. But when it comes to interconnecting hundreds of servers with different rules, moderation and legislation, and protocol versions, it becomes hell and no one wants to have to manage it (e.g. Mastodon).
There are objective reasons why these systems are not popular.
The other problem is that the very use of this type of software becomes a marker. I am convinced that the majority of Olvid users work for the French government, for example.
Iranian activists who are checked at the border or elsewhere with any uncommon communication application have already lost, regardless of the security of the application.
Crypto-punks are a niche group that can accept this type of usage constraint. My grandmother cannot, but she can use Signal and she will be one user among millions.