There are a lot of interesting reasons but for the purpose of this article the one I would like to highlight is the baggage of X86 backwards compatibility. For the first time power efficiency became more important to the success of a CPU than speed. All of the transistors and all of the millions of lines of x86 code that Intel and Microsoft had invested in the PC became an obstacle to power efficiency. The most important aspect of Microsoft and Intel’s market hegemony became a liability over night.
From the Apple iOS App Store Review Guidelines (https://developer.apple.com/app-store/review/guidelines/):
2.10 iPhone Apps must also run on iPad without modification, at iPhone resolution, and at 2X iPhone 3GS resolution
Moore's law is the observation that the number of transistors in a dense integrated circuit doubles approximately every two years. The observation is named after Gordon Moore, the co-founder of Fairchild Semiconductor and Intel, whose 1965 paper described a doubling every year in the number of components per integrated circuit, and projected this rate of growth would continue for at least another decade.