We've been through this over and over again - Smash season doesn't end on a bang, let alone one that rivals the peak of the reveals. Game & Watch, Olimar/R.O.B., Shulk/Duck Hunt, Corrin, Incineroar/Piranha Plant...there's a very clear pattern here, and only indefatigable hope keeps people believing the opposite will be true. Fighter #5 turning out to be a bombshell reveal would be the actual surprise at this point, but honestly I find it safer to expect an Incineroar-tier pick.
Truly, who really knows why Piranha Plant was picked over all other characters? Maybe Nintendo demanded a Mario newcomer from the mainline series. Maybe it was an Assist Trophy that got out of hand. Maybe Sakurai thought it'd be the most hilarious thing to do, foreseeing the rise of #PlantGang and such. I can't say for certain what got Plant into the game, but I can say that it wasn't historical value, plot relevance, fan support, or any of the other usual factors that the Smash fanbase long assumed determined all the newcomers.
Personally, I side with "hilarious". A Goomba or Moblin being playable wouldn't have been nearly as funny.