A very short analysis on the Smash 4 newcomers and possibly why they were chosen. By yours truly, no less.
- Animal Crossing series popularity. It's currently one of the biggest Nintendo series.
- Popular request worldwide. Was requested for Brawl.
- Purposely put in the game to surprise people, as Sakurai stated. THE oddball choice and only character to not have been requested and without significant popularity. Wii Fit was a major hit on the Wii, though.
- Very popular character from the Mario series since her debut in Super Mario Galaxy.
- Popular character among Western players.
- Token latest generation Pokémon. Chosen before the release of X/Y, but ended up becoming popular among Pokémon fans, so it was a lucky choice.
- Kid Icarus: Uprising was a major success and Palutena's popularity worldwide skyrocketed since then.
- Was an easy choice considering Bandai Namco's involvement. Also a major video game icon.
- Fire Emblem: Awakening was the most successful game of the series and was the most interesting character Sakurai found from the game. Both genders also ranked high in Japanese popularity polls.
- Originally planned as an alt for Marth, she is the most popular female Fire Emblem: Awakening character according to Japanese polls.
- Popular in Japan, where he was one of the most wanted characters for Smash. Xenoblade is also a notable niche title outside of Japan.
- Originally planned as an alt for Pit, he is popular among the Kid Icarus fans and one of the most popular Nintendo characters in Japan, according to polls.
- Token retro character. The dog has notability, especially among Western fans.
- Another quite popular Mario character. The Koopalings also have significant popularity among Mario fans, hence why they were likely made as his alts instead of regular color/texture swaps.
Any Japanese developer is, to be honest.
It's easier for them to conduct polls and whatnot in their own country than having to involve the worldwide affiliates.