I honestly don't know. I mean, that could have been Nintendo's criteria when they picked the DLC options to give to Sakurai to make the final decision on which 5 got in, but its also just speculation. My opinion is that Nintendo looked at a mixture of A) who was popular in the Fighter Ballot and B) what have been insanely popular releases in the last couple years.
Joker fits an insanely popular game worldwide (I mean, it directly competed with Nintendo's own Breath of the Wild and Super Mario Odyssey for Game of the Year on various Game Awards in Magazines and Online). I also wouldn't be surprised if there were a lot of Persona requests on the Fighter Ballot due to Persona 4 being very popular and Japan having multiple Persona 3 movies in Theaters the last few years.
We all know Banjo was a popular request worldwide.
And do I need to say how popular Dragon Quest is in Japan? Dragon Quest XI likely did very well in Japan and various Dragon Quest characters likely did well in the Fighter Ballot.
I mean, never say never, we did get Cloud, Joker, and Banjo, but it seems less likely for Sakurai to pick two Western characters when there are various characters that are more popular/Japanese (Sakurai likes to meet with and communicate with the creator of the characters included in Smash, 1st, 2nd, and 3rd Party and its easier to meet someone in your own country that speaks your own language). The only character I could honestly see them going through that hassle for is Crash Bandicoot since he was insanely popular in both the West and Japan..which I say as someone who doesn't really care about Crash Bandicoot.
I, personally, feel that the 3 most likely characters are Sora, Crash, and Heihachi or Lloyd Irving (or another Tales character).