The challenge is to balance the opportunities to download data from the SC against the on-board power required to process and compress the data. Storage capacities (e.g. SD cards) greatly exceed the daily download capacity of a typical CubeSat Kit to a single Ground Station (GS). Therefore you need either multiple GS’s to maximize downlink contact, or you need to process and compress data on board for the relatively rare GS contact. A 1U CubeSat does not have much power on board to spend all its time crunching large amounts of data … especially if a Pentium-class processor is required for this task.