If the World Government knew that the Strawhats were on Thriller Bark, why didn't they inform the marines? The World Government was worried enough about Moria to sent Kuma to Thriller Bark, but not inform the marines? That's weird right? Unless they knew the kinds of things Moria was doing on the island - but there are enough marines out there who would look away from that sort of thing at get the job done despite that...
The World Government didn't want the world to know about Moria's defeat. Moving the marine would probably attract too much attention. While the marine could probably move in silence, the World Government wanted to keep Moria's defeat a secret as much as possible, and besides, they thought Kuma is already enough to eliminate the straw hats (which he almost did).