I am unsure how it got so popular. Apparently, over 40% of people who aren't self-employed exist in a "business casual" dress code environment (in the US and other White Collar western nations, anyway).
It is also not considered a uniform. This is news to me, because 90% of the people I see on the train look like some variation of this:
Khakis and collared shirts can be cool. But no one seems to want to try, it makes me sad.