The irony is that Steve Jobs specified performance per watt as the metric that drove them to Intel in the first place. If Intel had simply kept improving that metric, ARM wouldn't be challenging them.

Around the same time, people in the server & supercomputer world were discovering the importance of performance per watt. Energy efficiency lets you run faster without overheating. The smaller the transistors, the harder it is to whisk heat away. Also, chips were getting cheaper but electricity wasn't, making the electric bill dominate the cost of a server farm.