Well, the space shuttle runs on only 1MB of RAM, all of the processors used in shuttles look conspicuously low power.
Wikipedia describes the Space Shuttle computer systems:
The shuttle uses five identical redundant IBM 32-bit general purpose computers (GPCs), model AP-101, constituting a type of embedded system.
The IBM AP-101 computers originally had about 424 kilobytes of magnetic core memory each. The CPU could process about 400,000 instructions per second. They have no hard disk drive, and load software from magnetic tape cartridges.
In 1990, the original computers were replaced with an upgraded model AP-101S, which has about 2.5 times the memory capacity (about 1 megabyte) and three times the processor speed (about 1.2 million instructions per second). The memory was changed from magnetic core to semiconductor with battery backup.
The Space Shuttle computer system could probably be put to shame by the processor that handles the encrypted GSM communications in a non-smartphone, let alone the fully GUI-ified sweetness that is a modern smartphone.
Most processors for space hardware have to be low power to be able to shed heat fast enough, the convection currents needed to remove heat naturally don't work in space, and using a motorized fan is just generating more heat to remove a little so just shifts the problem slightly (to all the components around the system that can't shed heat themselves). Generating more heat to remove some heat isn't something that can be done in such a small environment, heck, we're doing a good job at roasting an entire planet, let alone a small tin can in space.
I remember reading, but don't remember where, that the majority of processing for these types of systems is done groundside, with simple instructions on what to actually do being sent up to ships and satellites, so a satellite is a simple message relay with little or no active intelligence.