It is because Monte Carlo is not a Mandatory Masters 1000
From my memories, it was already like that in 2008, when Monte-Carlo was still mandatory : I guess Monte-Carlo has had a special status for that for some time.
No Murray, opposite to what has been said in many papers
Then he will probably be in Monte-Carlo, Del Potro probably not.
Many French players except the best one on clay, Monfils