The hardware has to be manufactured from the ground up for space. The silicon has to operate over a wider temperature range, the processor needs internal redundancy as well as external redundancy, and memory (caches included) has to include support for multi-bit error correction. Getting electronics to work reliably in space is no small feat.

And since the fabs that manufacture this kind of stuff are working at a relatively large technology node, it's non-trivial to design something like an i5 for space. Why? Well, Intel and friends have no interest setting up an entire fab process that will be utilized once in a blue moon. Even if you could use a more advanced node, the extremely high circuit area overhead required to realize the requirements I listed above will remain a limiting factor.