Nice isn't just a leisure route. There are a lot of trade fairs in the area, and a good smattering of high tech firms. Also the leisure traffic is definitely of a premium nature, with lots of Club passengers - my record is 15 rows on a 767, that's 90 Club seats, all of them occupied.
And don't forget the connecting market - I remember a BA manager telling me years ago that Nice had the biggest feed onto Concorde of any European route. I often see pax on NCE-LHR flights clutching onward boarding passes in First to the US, Asia etc.