http://www.gamespot.com/articles/playstation-cpu-powering-probe-travelling-towards-/1100-6428896/
...the probe contains a MIPS R3000 CPU, repurposed to "fire thrusters, monitor sensors, and transmit data."
The same CPU was used in the original PlayStation console, which was first released in 1994 and ran at 33.8688MHz. It was implemented by NASA scientists in the New Horizons Space Probe, which was launched in 2006 and has travelled more than three billion miles in its journey.
According to The Verge, NASA has always preferred hardware that has been "tried-and-tested" over newer technology, which may be riskier to use. Reliability is prioritised over power, which is why a 12-year-old CPU was used, with a few tweaks made to help with surviving in space.