Although, like mentioned in above comments, Signal does not hide metadata, which can be as valuable to attackers as the contents of the messages themselves.
Also, in terms of adoption, it's still hard when everyone has phones that use iMessage or SMS by default. iMessage is end-to-end encrypted, but not compatible with Signal, and Android has no equivalent baked in. Using Signal requires two extra steps for the user 1) downloading/installing Signal 2) knowing about it and caring enough to use it in the first place.
Also, in terms of adoption, it's still hard when everyone has phones that use iMessage or SMS by default. iMessage is end-to-end encrypted, but not compatible with Signal, and Android has no equivalent baked in. Using Signal requires two extra steps for the user 1) downloading/installing Signal 2) knowing about it and caring enough to use it in the first place.