The thing is that ballroom and Latin dances are really different to other forms of dance. Being a ballet dancer is a help but doesn't necessarily mean you'll get your head around a Charleston or a paso doble.
Being an athlete like the sportspeople that take part is an advantage because they can cope with the physical demands of training. Being a musician has an advantage because it makes it easier for them to follow the music and notice the rhythms. Being an actor has an advantage because they can perform and sell the act to the audience.
The ones who really struggle, unless they have natural talent, are the presenters, the politicians, etc. as they don't have any obvious transferable skills. It's also partly a popularity contest - the best dancers often don't win the whole thing due to the public vote being given priority over the judges scores.