While you're absolutely correct, I think it is interesting to note that your argument could also have applied to the HTTP protocol itself, given how widely HTTP is used.
However, in reality, the people/forces pushing for HTTP2 and QUIC are the same one(s?) who have a defacto monopoly on browsers.
So, yes, it's a political issue, and they just implemented their changes on a layer (or even... "app") that they had the most control over.
On a purely "moral" perspective, political expediency probably shouldn't be the reason why something is done, but of course that's what actually happens in the real world...
There are numerous non-HTTP protocols used successfully on the Internet, as long as they run over TCP or UDP. Policing content running on TCP port 443 to enforce that it is HTTP/1.1 over TLS is actually extremely rare, outside some very demanding corporate networks. If you wanted to send your own new "HTTP/7" traffic today, with some new encapsulation over TLS on port 443, and you controlled the servers and the clients for this, I think you would actually meet minimal issues.
The problems with SCTP, or any new transport-layer protocol (or any even lower layer protocol), run much deeper than deploying a new protocol on any higher layer.
However, in reality, the people/forces pushing for HTTP2 and QUIC are the same one(s?) who have a defacto monopoly on browsers.
So, yes, it's a political issue, and they just implemented their changes on a layer (or even... "app") that they had the most control over.
On a purely "moral" perspective, political expediency probably shouldn't be the reason why something is done, but of course that's what actually happens in the real world...