It probably depends on how popular they think the attraction will be as a whole more than likely. If they hypothesize that the attraction would be flooded with people in EP for an extended amount of time (meaning after weeks, or months pass) they might pass on the idea of giving the attraction EP.
Considering the fact that Gringotts single rider line often gets closed sometimes due to too many people using it as a way to getting to the ride quicker. After so many people get in the single riders line, you might as well just join the regular line, as the line itself doesn't move much since it's only to fill empty seats. Typically it is the quicker way to join the ride, but honestly, it doesn't take much for it to get to the point that you might as well turn around and go in the actual line itself. At least in that one you move at a regular pace. I can't fathom the amount of people who go into the line with the thinking they're getting there quicker, when in reality they're going to wait even longer if they have all the people in front of them in the staircase and they're behind even the place where they keep the glasses. If you're all the way back there, there's no reason to stay there. I remember one time I saw the line and walked back saying, "It's too long," when someone asked why I was going back. It was a group of people and one of the guys said, "Don't listen. You get on the ride right above the staircase." Um, yes, that's true and technically the line might be shorter, but what, you're going to have one person get out of line maybe once every five minutes, maybe two, tops three and you're almost at the beginning?
Sorry, kind of got off topic... That being said, I personally think that's why they chose certain rides to not have EP while they first open, because quite honestly, it would just be flooded with people if the ride itself is already insanely popular. It would especially not work due to EE, as anyone who has EE would more than likely have EP as well, since most EE guests are typically staying on property. It would just essentially make EP not so much an EP anymore.