What determines the endings of ethnic labels?

Question

By ethnic labels, I mean names for ethnicities like Spaniard, French, Mexican, Chinese.

By endings, I mean the "-ese", "-an", "-o" that follows the country they were named after.

An example being Korean. Why were Koreans not called "Korenese"? Or why where the French not called "Francans"?

Answer 1

How about the weirdness that Chinaman is considered insulting, but Englishman or Scotsman isn't?

Answer 2

I'm not sure whether you're the Confus-er

or the Confus-ee...