As someone who doesn't live in Florida, it seems to me that Nelson is a relatively inoffensive senator. Since nobody seems to have a problem with him, why should they get rid of him?
Maybe because of an unpopular Democratic president and the fact that 2018 could turn out to be a GOP wave, hmmm? Bill Nelson has always had so much luck (Democratic waves, terrible opponents, etc.) but 2018 might indeed be the fight of his life.
True, that could happen. But the fact that Scott barely won both of his gubernatorial campaigns against not-so-great opponents leads me to believe that if he were the nominee, Nelson could probably defeat him without too much trouble.