"Both the words and the melody make it an infinitely better song than 'Flower of Scotland'". You forgot to add, "in my opinion".
I actually happen to agree with you, but what % of the Scottish population [ a relatively low % of which know the words to "Auld Lang Syne", if recent reports are to be believed ], could quote even 1 line of "The Freedom Come All Ye". ? Not much of a basis for a national anthem, I would suggest.