Engineering conservatism and flight heritage are definitely part of the reason that spacecraft processors tend to be antiques, compared to mainstream commercial hardware. However, there are also solid technical reasons.
Digital processors fundamentally work by turning transistors on and off. These days, the transistors are MOSFETs. One of the fundamental limits to how fast MOSFETs can switch on and off is gate capacitance: the higher the gate capacitance, the longer it takes the gate to “charge up” and switch on. There are a couple ways to improve this. First, you can make the transistor physically smaller, directly decreasing the gate capacitance. Second, you can design the transistor to operate at lower voltages, so that fewer coulombs have to be shoved into the gate to turn it on.
Altogether, this means that as time goes by, processors get smaller and lower-voltage. Shrinking the size (e.g. “14 nm process”) allows you to fit more transistors on the same area of silicon, decreases the gate capacitance, and decreases speed-of-light delay (although that’s a much less significant factor). Similarly, CPU core voltages are decreasing: 5 V and 3.3 V used to be common, then went to 2.5 V, and are now down to 1.6 V or so.
Unfortunately, all of these things negatively affect radiation tolerance, one of the key requirements for space electronics. Radiation primarily affects electronics by ionizing an atom, and thus depositing some charge. If this happens near a MOSFET gate, the charge can change the state of the MOSFET, leading to effects like SEUs, or worse, latchups. Any changes to a MOSFET which decrease the amount of charge needed to change the state – such as shrinking it, or designing it for a lower voltage – mean that it will be more sensitive to ionizing radiation.
Thus, in addition to sticking with designs like the PPC750 because they’re tried and true, older processors are also quantitatively more resistant to radiation than newer ones.