They need a game as a test bed. If there's demand for a game that they can test with and theres a success, that's when they'll port others.
Well, I don't really test beds are ever that useful, because the truth, especially on PC where word of mouth matters a lot, is that every game performs on its own and you can hardly predict what is going to be successful when you are basing your stats on a previous game you published which was also entirely different.
I'm not sure how they could grasp anything useful about the potential market for Valkyria Chronicles or Persona 4/5 looking at how Vanquish or Bayonetta performs, for instance.
Edit: Also could you post a link to some of the port data?
No, because I'm not talking about something circumstantial or specific, I'm talking about years reading blogs or posts form developers or talking to them myself and pretty much the scenario is always the same: you have from one to six people working for six months at most to port most of the games.
Maybe some special juggernauts (GTA IV/V?) could occasionally require a lot more, but we all know that's not what we are talking about here, right?
You aren't expecting SEGA to re-do the texture work for Valkyria Chronicles, Bayonetta or Vanquish entirely, right?