I have flown the MCO-ATL run many times. In addition to the high demand on the inbound flights from ATL, some of the MCO-ATL widebodies continue on to Europe. I took a 777-200 (MCO-ATL) in early 2001. We arrived in Atlanta at a gate in the international arrivals area. The passengers not continuing on to London Gatwick deplaned, and the flight left ATL bound for LGW shortly thereafter. A good number of the passengers on the flight originating in MCO remained aboard.
With that, you could say that another reason why there are so many widebodies running the ATL-Florida routes is that they ferry passengers through the ATL hub en-route to Europe.
For the record, Delta is the largest carrier serving MCO (in terms of the number of enplaned passengers per year).
Chris in Orlando