I would see three potential use cases in which a developer would consider porting their smartphone game.
1.) Their game is notably successful among young children, many of whom do not yet have a smartphone. Gung-Ho and Mixi have been doing a nice job showing that making alternate versions of the games is probably not the world's greatest long term strategy, so I would expect the f2p versions to show up so the kids are hooked by the time they get a smartphone and are more likely to spend money (or just to encourage them to buy all the merchandizing even if the product itself isn't earning much).
2.) The game is something that's both being played a lot by the type of people who are dragging their handheld around everywhere anyway, and would at least be a natural fit for a traditional control scheme. Like, if Colopl determines that people who play Monster Hunter have a 30% chance to also play Rune Story on their phone, they might consider porting that game over since it's a pretty natural fit for a handheld, and their enthusiast audience could end up spending more money if they're playing it on a preferred platform (assuming all their progress transfers).
3.) The title is essentially a handheld game that's being made on a phone. If you look at something like Adventures of Mana, that would fit the mold.
Overall, I feel this applies to a pretty slim selection of mobile titles. I would expect to see very few mobile games ported over compared to the total volume of mobile games being made. That said, I wouldn't be shocked to see at least a handful of titles each year show up on the platform whenever they make sense.