Pit and Sonic were more popular in the West at the time of Brawl's release. Actually, Sonic really isn't that big in Japan to begin with. You can see that Sega has attempted to shift the franchise's appeal to the Western market by replacing the anime-styled art direction, plot, and characterization of the Genesis and Adventure games and replacing it with a more Pixar-like aesthetic and writing style. I have my doubts that Sonic would have been in Brawl had his massive Western fanbase not been taken into account. Sakurai himself made note of his decision to include Sonic having been influenced by Western requests.
R.O.B., while not exactly popular, is much more significant in his/its role in the Western market, so that should be considered as well.
I think that for the most part, Sakurai has been very fair thus far. He's catered to the West (Sonic, Pit, ROB, Diddy to an extent) as well as the East (Ness, Lucas, Jigglypuff). In fact, he's probably been more generous to us Westerners. I do think that there is still no way to tell whether Sakurai will continue with his precedent or instead focus on one region more than others this time, however. A lot of the most popular newcomer candidates, like Ridley, Little Mac, Isaac, and possibly King K. Rool, are largely more popular in the West, almost exclusively so. I'm not so sure that Sakurai will cater to us so willingly. I'm sure he wants to please the Japanese fanbase at large as well, so I'm hesitant to believe we'll be seeing all or even any of those characters.