Because there are two distinct types of radiation effects in space: ionizing and upsets. Multiple computers voting solves the upset problem, but not the ionizing one. Ionizing is a measure in krad of how much radiation a part can be exposed to in its life and keep working. Ionizing radiation changes semiconductors in fun ways (e.g., gate voltage thresholds shift). They are fabbed in exotic ways like silicon on sapphire. When you want a satellite to last 20 years at geostationary orbit, you account for this! For example, I believe the Juno probe to Jupiter's life will be limited by ionizing radiation.