Probably because the word "c*ck" in this sense means "chief" or "most important". The lead horse in a team of 2, 4 or 6 hauling a wagon was called the c*ckhorse - hence the Banbury Cross nursery rhyme. The plume in a military hat is called a c*ckade because it sits at the very top of the uniform. The biggest feather on an arrow - the one which has most influence on the arrow's flight - is called the c*ck feather. And so on.......
Incidentally, I find it very interesting that AB has censored c*ck in all these messages, but left it intact in the title........
In Old Teutonic - from which many English words come - the equivalent word was 'kukko', from the same source as the one from which we get 'chicken'. Notice the opening 'c' and later 'ck' remain the same, though the central vowel has changed. The Latin word 'coccus' also existed. (In case that gets deleted, it opens 'co' and ends 'us' with 'cc' in the middle.) The answer to the question is, therefore, simply "linguistic history".
Roosters is a prissy American word they invented because the good old Anglo-Saxon word **** was too shocking for their Puritan minds. I noticed Anne Robinson saying The Chinese year of the Rooster,how pathetic.