I feel like the opposite is actually the case here; OBB 2021 sold out about as fast as it did in 2019 (2ish months, give or take a few weeks), but they didn't highlight the new villains until shortly after the party already sold out (Sid, Cruella, and Agatha were teased like a day or two after it sold out, and Zombie Cap like in the last week of the parties) so we never saw how the new villains would have affected ticket sales.
OBB selling out within days feels like a direct response to the additions they made last year. Folks likely didn't want to miss out on what new villains they might bring out this time around; already now that they teased Madam Mim and Mother Gothel as new villains, and I doubt that's the only surprise they'll be sharing since it's still so early. (If they don't bring out the Sanderson sisters with Hocus Pocus 2 on the way, something must be seriously wrong with the D+ marketing budget.)