Hard drives, no matter how small, have moving parts in them. Usually a rotor, a platter, and an arm that moves over / between different areas / platters when different parts of the disk are accessed. Because of this, a good amount of battery power is required to do the above. With present day battery technology, it becomes very difficult to provide power for an LCD screen, internal RAM, and processor for a reasonable amount of time before a charge is required. Compact flash memory is very battery friendly since it is solid state (no moving parts) and requires something like 5% of the power required by comparable disk drives. The physical limit of compact flash technology, as of now, is around 137GB but that'd be an incredibly expensive storage card. Point being, compact flash is much more expensive per unit of storage but much more battery friendly.
iPods have the luxury of not having a lot of what PDAs have so they can "afford" the added battery load of a rotating storage device.
