Hurricane Dean happened and shut Costa Maya down for a while. You are correct - Carnival did use to go there from New Orleans.
Hopefully they will return and/or at least alternate Progreso and Costa Maya.
It does take more fuel to get to Costa Maya than Progreso, but as long as they are going to Cozumel anyway?
I have been back to Costa Maya since Hurricane Dean, but it was on NCL from New Orleans.