The nearest I could find is from April through October, or November, depending on which ship, the Legend, and the Liberty offer an 8 day from Ft Lauderdale, with a stop at Colon Panama. They DO NOT go through the canal, but you can take an excursion that allows kayaking in the canal! WOW,,,how cool is that?
Purely speculation on anyone's part but I highly suspect it has something to do with the fees for entering the canal. We were on the Coral Princess about a year ago and did a partial transit ( enter Gatun Locks, cruise around Gatun lake and then exit through Gatun locks ) and according to the captain it cost over $ 200,000. It has to do with weight, size, etc. I think If i were Carnival I would keep my ships out too.