A Japanese game sold worldwide to an audience consisting of a lot more than Japanese people. Nintendo wants money and the vast majority of their audience would eat up characters that Japan might not care about. They've already tested the waters with Microsoft, who's to say it ends there?
I'd love for more Western characters to come to Smash, but between Sakurai outright stating how much easier it is to work with people he can understand/not separated by oceans, to their own biases understandably making them more favorable to Japanese characters, I'm uncertain how many more will get into this pass. I think it's important to bring up that historically, Japanese games have done quite a bit better in the west than Western games have done in Japan. There are simply more Japanese characters that the average person might at least
recognize in say, America than the reverse. I guess it's a bit of a situation where, say, Doomguy would absolutely sell gangbusters in America at least, but hype in Japan might as well evaporate in that case. Smash has tended to walk a bit of a middle ground in that regard - sure most of the characters originated in Japan, but a lot of the DLC we've been getting
for the most part has been reasonably popular in both countries, even if there's a heavy lean towards one side or another if you look at each individual character.
I don't think that it's necessarily the fact that Western games have some sort of inherent uphill battle to face because they're Western more than the fact that (older) Japanese characters tend to be more well known in both areas than the reverse. And the western characters that have come to Smash have been well known on both sides - Banjo did really well in Japan when it came out, and Minecraft doesn't need much of an explanation.
(You know, in hindsight, I think this actually makes Master Chief if not the most likely character, definitely very much possible - Halo 3 did
waaaaay better than you would expect an
Xbox exclusive FPS to do in Japan.)
(Of course, this trend is starting to change in more recent years. Putting Minecraft aside, games like Fortnite are also very popular in Japan - as very much annoyed I would be with... Jonesy, for example, the only real argument you can make against him that's not some variation "cringe" or "he's not really talked about that much" or "zero leaks" is that he's too recent or something like that. You cannot convince me that an army of 10 yr olds will not come descending on Jonesy to default dance on their friends in Smash.)