Facts of the matter are saying "Rugby League", "rugby union" and "australian rules " is a mouthful in general conversation, so this is why everyone just says "football". Except rugby union of course who now have a mortgage on rugby and therefore can use that.
It comes down to the number of syllables, three is too many, two you can get away with, unless of course you are at a Donald Trump rally where three syllables works perfectly fine, especially when yelling out things like "lock her up", " drain the swamp" or even " build that wall".