I suspect part of it is licensing games, both in the sense of "avoiding per core license limits" which absolutely matters when your DB is costing a million bucks, and also in the 'enable the highest PVU score per chassis' for ibm's own license farming.

Power systems tend not to be under the same budget constraints as intel, whether thats money, power, heat, whatever, so the cost benifit of adding more sub-core processing for incremental gains is likely different too.

I may have a raft of issues with IBM, and aix, but those Power chips are top notch.