Why doesn't Nintendo just make a Pokemon MMO? I'm sure they could get away with a subscription model.
It'd almost certainly cost a shitload of money to develop, a shitload of money to maintain, and MMOs tend to be extremely risky nowadays to begin with.
With mainstream games, however, they can give GF a much smaller budget, would only have to pay for some servers (as opposed to GMs, developing new content to keep players playing, etc.), and new mainstream games will ship like 15+m while third versions/remakes will get at least 7m.
And yes, even a Pokemon MMO will be risky. Most Westerners spend most of their Pokemon time by themselves, even if they do occasionally dabble in trading or battling. In other words, a big part of the appeal in the West is the single player campaign. Meanwhile, in Japan there's a thriving local play scene in addition to single player. In other words, there's no guarantee that either of these audiences would be thriving for an MMO.
Adding online features to the mainstream Pokemon games is an infinitely safer route to take, really. I don't blame them - in recent memory most MMOs have traditionally not gotten traditional Pokemon-level profits. And no, slapping Pokemon on something doesn't guarantee amazing sales. See: Pokemon spin-offs.