Marvel has filed to claim the Miracleman trade mark and already own Marvelman. Which makes sense, as the character has appeared under both names.
I'll idly speculate that one name can be used to continue the story started by Moore, while the other can be used to bring Marvelman into the Marvel Universe.
The most powerful hero in the Marvel Universe is a Brit!!!!!
Realistically they just don't want anyone else using the name.