Airs are different because they were mostly sung a cappella in any key or mode the singer wanted. Musical instruments were something of a luxury back in those days. The "muckle sangs" or old ballads were by their form and repeated second and fourth lines and choruses obviously meant to encourage the audience to join in. Where words were put to existing tunes taken from the bagpipe repertoire, you end up with a lot of pentatonic songs. Also the pitch of the drones on the highland bagpipe fit in with the pentatonic scale. Other notes on the regular 8-note octave tend to make a dischord. (Although some would say, on the bagpipe, how could you tell?)