TL:DR: Young people cannot afford to live in the big cities and are moving where they can live and not work themselves to death.
Wow, it is almost like quality of life is something people care about.
There was a time when I thought I wanted to live in a big city. That time has passed. I can't imagine anything that would draw me away from my mid-tier city. Among hobbies I have, the only thing a major city would provide is high likelihood of major touring bands playing there. Anything else, no.