Dixie, K. Rool and Captain Toad are the last "old" A-list Nintendo characters yet to be in Smash. If Sakurai decided the roster in their late 2015 or early 2016, than Tropical Freeze and Captain Toad were among the most recent releases...and K. Rool likely did well in the ballot.
Captain Toad first appeared in Super Mario Galaxy, which was released in 2007. How is he an "old" character?