It's because everyone knows the Lyrics are out of date. That bit about "rebellious Scots to crush", for example. No longer relevant today. I mean, when was the last time the Scottish people tried to rebel against the UK?
I've seen and heard many sing it in my lifetime and certainly lots in the past couple of weeks - some people may have their reasons not to but that's their choice.
It's the same with the Welsh anthem. Although many will know the words to the first verse of 'Mae Hen Wlad Fy Nhadau', very few will know the second and third verses as they are very rarely sung.