Up front Business
There are a number of 777's in the BA fleet which do not offer 4 class operations and are used on MCO.
LAS has a number of industries based there which have connections with the UK - Lockheed Martin, Raytheon, Microsoft (Reno) Otis Elevators, GE, Honeywell, Ingersoll Rand etc etc.
So there is a level of business traffic and these are only the obvious ones. I'm sure some of the gang at BMI or VS might be able to hint at the upfront loads. Also, being a gambling capital there are 2 types of gamblers, 1 of which enjoys to have all the luxuries, will stay in the Penthouse at Bellagio etc. So, while they are leisure passengers they will pay premium prices.
VS and BMI wouldn't upgrade the routes unless they felt it was worthwhile especially with 747's and A340's.
Shamrogue