The only valid (but not raised yet) argument is that something like this could endorse nostr and I don't believe they want to do that. Protocol or not, there is not enough surface area of products that have reached mainstream for it to be considered a useful protocol that should be incorporated. It's like deciding to integrate a remote file sharing protocol when there are 3 others to choose from and so you'd wait for it to become 1 or for at least one of them to become dominant before integrating.

Yet building your own protocol seems less troublesome even though it is technically the same thing. Like how Telegram made their own blockchain protocol and endorsed it on their app.