Unlikelhy, event that there would be a first-term governor being taken out of office and Dems do have Cory Booker.
Gillum is younger, comes from a state that's more important in the presidential election, doesn't have weird Wall Street controversies, and has a wife and kids.
A good chunk of the potential presidential candidates come from the Northeast (Biden from Delaware, Sanders from Vermont, Gilibrand from New York, Warren from Massacusetts) so he's a better geographic fit as well.
Weirdly, the past several waves of Democratic Veepstakes (in 2000, 2004, 2008, and 2016) have all tended to concentrate on more experienced figures for one reason or another; Hillary did consider some admiral who hadn't sought elected office but otherwise it's almost all experienced politicians. The most recent freshly-elected last-cycle Governor/Senator to be a Veep finalist was John Edwards for Al Gore in 2000.
Whereas Republicans like to do this sort of thing a lot more -- see Romney considering Rubio, McCain actually choosing Palin.
Democrats almost always go with Senators, and the main exceptions (1972, 1984) were landslide losses with unusual circumstances in the veepstakes.
There's a variety of explanations (Senators have a combination of appeal to statewide voters and knowledge of national issues, they represent government to a party that's okay with it, the lack of presidencies from 1969-2008 limited the number of cabinet officials to nominate for Veep)
Why do people keep suggesting Democrats running for governor in 2018 in states that aren't blue for VP? That would be an incredible waste.
Florida was pretty close in 2016, so it would be helpful. There could be also be the hope that he helps in other states.
