Stroustrup quotes it as not enough resources, and I can really picture that especially because of what you say. When people disagrees, it will take a tremendous amount of time to reach an agreement that works with everyone. It just takes too much time.
No, actually it's lack of agreement. Take any two people from the C++ community and ask them what their GUI library should look like. You will get two answers that are
1) either incomplete (too little functionality to be useful), infeasible (feature creep, too big), or unportable (most common problem) and
2) very different.
Stroustrup has pretty much said that nobody needs to bother sending in a GUI proposal because it's just not happening.
Think about the devices C++ targets: everything. Do you really think that you can design a GUI library that is equally applicable to desktop PCs and handheld devices? Have you ever looked at design paradigms of the iPod Touch and compared them to the way desktop software is designed? Do you think the two can be unified?