Aerospace grade radiation hardened computers start in the hundreds of thousands of dollars per CPU and with a cost equal to or greater (usually the latter) than the launch cost, they just aren't worth it for CubeSats that aren't carrying critical payloads (piggy back launches are $200-500k depending on institution affiliation).
A little shielding is enough to protect the CPU from permanent damage for a while and error correction circuits using 3-7 or more CPUs running in parallel is way cheaper, even including engineering costs, than a single CPU.
Even with the troubles, one hell of a success for $70,000.
