If the World Government knew that the Strawhats were on Thriller Bark, why didn't they inform the marines?
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).