im pretty sure that if "chromax" was indeed more popular, they would abandon their brown theme. Yet since the brown continues to be dominant, you comments on the black/grey being more popular is a little inaccurate.
And im pretty sure (guessing here) that the mainstream user still builds a case with zero care on aesthetics, having no PC case window, no WC, no interest in tidiness inside and chooses whatever they can get their hands on, or has no worry on theme/colour of components.