Greedy, selfish companies.... something something why not Patreon something something.... how does business work.
I think that about covers it.
In all seriousness, Dante wasn't in the "original" release, he was in the "director's cut" version which is the one we received in America, and he wasn't even in the "super director's cut" that, like the original, only came out in Japan. So up until this recent game, Dante was only featured in 1 of the 3 different iterations of SMT3.
He was a licensed character from Capcom and probably only licensed for that one game. To get him in this upcoming version, Atlus probably had to re-license him from Capcom again, which means $$$ (or in this case ¥¥¥), and so, like the first poster above, I'd rather have him as a payable option than no option at all, though $10 sounds a little too steep for my blood.