Because they sell better, frankly. That's what I would assume, at least. I know that most of us would just like MS to fill in the cracks of ML, but can we really expect Diamond to think that way? It's not much of a strategy.
I don't say this as a MS fan; I own exactly one, Mephisto (although I really
wish I'd bought Thanos when he was around...
). But I understand the logic behind DST wanting to make the more iconic characters, particularly now that ML is on it's last legs.