Globalization plays it's part, but I think the real root cause is the shift in business philosophy to systematically squeeze margins for short term goals. Worker salaries are one vulnerable facet there, but between businesses margins are tighter too. It basically optimizes efficiency & profits today, but edges out a number of avenues for better long term growth - larger investments in new products within companies as well as the worker pay & savings (which in turn hurts for example self-funded small business starts).

In theory, tighter margins means better purchasing power for everyone, but somehow that excess from the squeeze seems to be unevenly sticky as capital flows past the upper wealth brackets.