Moblin is a Linux distribution, that happens to have a different look and feel (eg Desktop environment). Some other Linux distributions have also decided to ship this environment, and call this "Moblin", without fulfilling the compliance requirements that Moblin has (in order to call something "Moblin" within the trademark rules etc), while others do a "real Moblin" including being meeting the compliance requirements. These compliance requirements are to ensure that apps written for "Moblin" run on all things that call themselves "Moblin". (think of this as a "super LSB")
Now in the MeeGo context, it turns out that the Moblin compliance was not nearly strong enough for application writers, so MeeGo has an even stricter compliance requirement rule set.
As for Canonical currently not doing MeeGo (Canonical has publically stated that they'll do a MeeGo version if an OEM requests it and pays for it); it's never easy to merge two operating systems, and unless you go all the way and spend the effort, you run the risk of having the worst of both worlds, not the best of both worlds. I can understand Canonical not doing this lightly... it's a really hard task.