I'm guessing they locked the internal resolution for one (or all) of the following reasons:
1) The UI was designed for 1024x720 and doesn't scale nicely. 2) The game is poorly optimized and the devs didn't have faith that they could maintain performance at higher resolutions. 3) They didn't have the QA budget to do resolution testing. By limiting the resolution and framerate to that of the console versions, they probably figured that would reduce the amount of testing required.