I worked for Intel for a few years, 2005-2007. It's hard to put into words what happened to Intel. The simplest way for me to explain it is that the employees didn't take risks. A different way to explain it is that most people at the company were too career focused then focused on the actual business.
But this is how disruption happens! Major players like Intel can always see where the industry is going, at a high level, but the rules of each generation in an industry are always murky.
IMO, the "ARM" generation of PCs is a shift to vertical integration. Intel set itself up for horizontal integration, and the openness of ARM means that the lines between "Computer manufacturer and CPU manufacturer" are going to increasingly blur.
Intel's business basically treats PC manufacturers like franchises; and Intel's a culture focuses more on career, and less on risk and business. This means that they're going to increasingly have trouble adapting to the new rules that come in the "ARM" generation.