1.) some were closed-source and proprietary
2.) some were needlessly verbose, or enforced a workflow that made it difficult to interact with designers
3.) some depended on HTTP for at least the initial handshake
What the world needs is an open-source GUI technology that is fun for designers to work with and which uses a protocol other than HTTP.
My own bet would be that when we finally get such a technology, it will come from the world of gaming, where they already do amazing GUIs, and where they often abandon both HTTP and TCP and use UDP instead.