Batteries are the reason PDAs have the limitations that they do. Laptops can afford to have bigger batteries, because they are expected to be physically larger and heavier. There's only so much energy that can be stored in a battery of a given volume and weight. To make a PDA small enough and light enough to be useful, the batteries need to be small.
Since the batteries are small, the amount of power available is severely limited. Large screens, fast I/O, large storage, and fast CPUs need power that just can't be squeezed into a tiny battery. So compromises need to be made. Laptops get perhaps four hours of life these days out of much larger batteries. PDAs get days or weeks out of much smaller batteries, because the have to.