Historically, the purpose of a trademark is to identify the source of a good or service. From a trademark perspective, “*buntu” names are a bad idea. The two components of an *buntu name are each trademarks themselves. Without getting into details, smooshing together two trademarks creates the risk of potentially diluting each of the underlying marks.
Anyway, whether Canonical’s current policy stems from consultation with a trademark specialist, or is based on some other reason, as @popey previously mentioned, Canonical is no longer receptive to new *buntu names.
And in any case, “Ubuntu” and “Uubuntu” would sound too similar (identical?) when spoken, and so the two names would lead to confusion.
Given the above, one may wonder why I guess that Canonical would find either of the first two logos that I submitted (the ones that include the full Ubuntu logo, combined with the Unity “U”) acceptable. Well, what Canonical would decide, I can only guess. But I know why Canonical should find them acceptable. The explanation would take to time to craft. If there is any genuine interest in my reasoning, let me know, and I’ll do my best to explain.