There’s no reason to ask everyone on the planet to change “https:” to “http3:” when we still can’t even reliably complete the “http:” to “https:” transition. We’ve learned that users simply do not care at all about “http:” or “https:”. We shouldn’t have to ask people to update their QR codes for HTTP3 just because we updated the protocol.
Should we have introduced ftpp:// for passive FTP to distinguish it from classic FTP? No: it would only confuse users, cause frustration for pages linking to FTP urls, and both servers and clients are perfectly capable of negotiating this silently without surfacing it to the user.
In general the pushback with HTTP3 is that site operators would rather not have to do the extra work to enable it. But from a user’s perspective, there is no work to enable it. It’s just https:// like always and one day it gets faster. Sites that refuse to do the extra work to turn it on will be visibly slower than their peers once it’s widespread enough.
If you believe that HTTP3 should have had its own URI spec, you would have needed to make that case a few years ago to the committee implementing it; it’s not going to change now. I assume their discussion about that is in the archives, and I expect it boils down to “this is not relevant to users, they should not be expected to care”.