Lucas and Roy were gambles on how they would sell imo.
Both were Japanese only characters and were vets of Smash, but not particularly popular when they were actually on the roster.
And even then, I kind of wonder how they sold. A few months later, and Ryu is the only DLC character that gets any consistent tournament play, and even then, it's not much.
Obviously Smash DLC is going to sell well regardless, but it would be interesting to know just how well specific characters were.
I'd guess Mewtwo was the least bought, pretty much entirely because of the promotion. And Ryu being the most bought because he comes with a stage and is an entirely new character, while Roy and Lucas are probably somewhere in the middle.