Power constraints and heat dissipation constraints.
https://arstechnica.com/science/2019/11/space-grade-cpus-how...
> BAE RAD5545 is probably the most powerful radiation-hardened processor available today. Fabricated in the 45nm process, it is a 64-bit quad-core machine clocked at 466MHz with power dissipation of up to 20 Watts—and 20 Watts is a lot. A Quad Core i5 sitting in a 13-inch MacBook Pro 2018 is a 28 Watt processor. It can heat its thin aluminum chassis to really high temperatures up to a point where it becomes an issue for some users. Under more computationally intensive workloads, fans immediately kick in to cool the whole thing down. The only issue is that, in space, fans would do absolutely nothing, because there is no air they could blow onto a hot chip. The only possible way to get heat out of a spacecraft is through radiation, and that takes time. Sure, heat pipes are there to take excessive heat away from the processor, but this heat has to eventually go somewhere. Moreover, some missions have tight energy budgets, and they simply can’t use powerful processors like RAD5545 under such restrictions. That’s why the European GR740 has power dissipation at only 1.5 Watts. It’s not the fastest of the lot, but it is the most efficient. It simply gives you the most computational bang per Watt. The HPSC with 10 Watt power dissipation comes in at a close second, but not always.