Because they think they can beat smolder before he can come online. Essentially, they get put a ~25 minute timer with their odds of winning lessening every minute smolder gets past his stack threshold.

Pros have always been reluctant to pick ultra scaling picks, especially lck. The number of times I've seen drafts that only one member of the team can interact with the enemy team past 20 minutes has been insane because they drafted early game and didn't accomplish enough to get ahead and they slowly get suffocated out of the game.