G'day pipe-lovers ...And it has been played for ever (well, since Sir Henry Bishop wrote it ...) by the Pipe Band of the Royal Scots Dragoon Guards because it is their "official" hymn - with a pun on the Royal Scots Dragoon Guards' nickname "The Amazing Greys".
As well, any tune with a short enough range for Highland pipes will also have a short enough range for most voices to join in without neding to create a harmony line on the fly.
Regards,
Bob Bolton