They need to use radiation hardened processors. That means the chip will be in special packaging (I think ceramic), use bigger process geometries and, use redun

They need to use radiation hardened processors. That means the chip will be in special packaging (I think ceramic), use bigger process geometries and, use redundancies in logic design within the chip. Also running at lower clock rates will reduce the chance of failure. These are not mainstream chips so they will be expensive and slow development. And lastly certification takes a lot of time and there are risks to the mission to switch from a proven technology to something new. A single glitch can end a mission that took a decade to accomplish.