I thought it was popularized by internet/Reddit as a joke denoting that ginger cats are "special": https://www.reddit.com/r/OneOrangeBraincell/

No, they've been orange since long before that. Cause, y'know... that's their color!

Turns out it's a dead tie in English books though, wow: https://books.google.com/ngrams/graph?content=orange+cat%2Cg...

It's because they have no soul!