Makes me wonder how on earth did Nelson get elected in 2006 and 2012 then.
White people.
Nelson won a ton of rural counties in 2006, and didn't get demolished in them six years later in 2012. Nelson also outperformed Obama big league in 2012 pretty much everywhere, especially in what seem to be whiter areas (based on their voting patterns in presidential elections).
Also, all of Nelson's past opponents have been congresscritters from solid GOP districts. Rick Scott, meanwhile, has name recognition across the entire state, and is the incumbent governor. That is a undeniable reason to consider him a solid challenger; regardless of the margins he has won by, Scott seems to be relatively popular, and won in the state at-large.