Battery.

If you ever wrote animation you'll know the penalty for smoothness. Most of the times, especially with acceleration and large data (like dragging images and windows), CPU usage might well quadruple for double the frame rate.

If it was 60 fps instead of 15, that would most likely ceil the CPU and simply moving about the device would double or triple consumption.

It's no longer a tablet if fiddling with it was a major drainage. With phones it's not so bad, because people pull them to sms or call, but I fiddle with it endlessly, my battery would drop like a brick.

Oh, and, I very much doubt that the device could output uber-smooth anyway, seeing as this isn't a multicore and the screen isn't a 1 ms wonder.

As a last note, animation is tweakable, should you wish to waste resources.