The two biggest limiting factors are power consumption and cooling. Smartphones are battery powered and passively cooled (they have no fans). So they favour efficiency over raw power.
Heat is essentially wasted electricity, so power consumption and temperature regulation go hand in hand. This means basically that different architecture CPUs are better in different use cases. In PCs we have a more powerful energy source and we use that for active cooling with fans or water and so we focus on raw speed at the expense of heat generation. In mobile phones we mostly want long battery life so we have to settle for less CPU speed.
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