In the early years, the SPUs were not all functional due to the fabbing process. The ones that had all 8 functional ended up in servers, and the ones with about 6 ended up in PS3s. This still happens all the time with clock speeds, and core counts on modern processors today. I’m sure the fabrication process improved over time, but they disabled the 2 cores to maintain backwards compatibility.

Unrelated: Every time I’m reminded about Cell I’m reminded of the OtherOS fiasco. I purchased a PS3 for the processor solely and I was very upset when I only got a $2 check for it. I never cashed it.