Well, this is pure speculation of course; but the x2 feature for iPhone apps was never more than a stop gap to help make up for the relatively few iPad apps that were available at launch. Now that there are tons, and Apple has supplied developers a way to support both devices with one app if they want too, Apple isn't likely to give a high priority to making the x2 feature work better.
They would probably prefer that the developer include the extra code to make the app universal. I would too, since that usually means an interface that works better with the iPad. Even the slight changes in the iPF app are better than running the iPhone version, no matter how crisp the display might be if it would adjusted to use the retinal graphics.
In short, Apple likely thinks they have better things to work on. Despite how it may seem to us, their resources are not infinite, and people who are working on one software improvement for an old stop gap measure aren't working on other things, like maybe getting AirPrint right.