So if the selections seem like they were decided by the marketing department... they were.
Masahiro Sakurai has issued a statement on the decision process for deciding which characters will be a part of the Super Smash Bros Ultimate Fighter's Pass.
In the two English language posts on his Twitter, Sakurai confirms that the decision on who the characters will be is made by Nintendo, and then they set to work seeing how to adapt the character to Smash Bros. Additionally, he called for restraint in flooding his replies with character requests.
Super Smash Bros. Ultimate’s DLC line-up is now complete. This time the selection was made entirely by Nintendo. I decide if we can create a fighter based on their selection, then come up with the plan.— 桜井 政博 (@Sora_Sakurai) November 6, 2018
It’s great to dream about your favorite character joining the battle and I appreciate your passion, but please try to stay on topic when replying to tweets and refrain from flooding us, and other users, with requests when we're not specifically asking for feedback. Thank you!— 桜井 政博 (@Sora_Sakurai) November 7, 2018