Signal is not a company but a non profit, and their service is not proprietary but fully open source including the server side. That being said, it is centralized and so less resilient, it can be taken down more easily. So you have to pick between more secure (Signal) or more resilient because decentralized (DeltaChat). Theoretically Matrix has both, but at the moment it is not as secure as Signal, and its UX is clearly worst. And to that you have to add the complexity of decentralization for normal people: which server to pick, how can I know if someone I know has an account... Here the comparison with email should help but still it is not as easy as entering a phone number and immediately you have all your contacts available.

when you say “Matrix’s UX is clearly worst”, what app are you talking about? Element X is similar if not better to Signal in terms of UX for instance.