You have to understand that most space missions are essentially webcams attached to a remote-controlled vehicle. You don't need hardly any onboard processing 99% of the time if you're just executing a sequence of actuator commands and maintaining PID loops.
It's only very recently that they've started to look at what the sensors are giving onboard the craft. I'm glossing over some very important details, but mainly, the sentiment holds: Spacecraft didn't have to do much, so they didn't have to think much.
The most intensive work spacecraft used to have to do would be to apply a (lossless, hopefully) compression algorithm to send more data over a limited high latency link. I wonder how much forward error correction capacity they allow for.
It's only very recently that they've started to look at what the sensors are giving onboard the craft. I'm glossing over some very important details, but mainly, the sentiment holds: Spacecraft didn't have to do much, so they didn't have to think much.