I've always joked that a marching band needs to know only two tunes...because by the time they repeat the first one, they're well past the people on the parade route who heard it already.

I would venture a guess that those are the two most popular tunes and that the majority of pipers who play in bands know them and can play them rather competently. There might be a certain percentage of the band that is competent (at least) with lots of tunes, a certain percentage that have a slightly more limited repetoire and so on and so on...if they want to have everybody playing, they'd pick the tunes that everybody knows...thus, Scotland the Brave and Amazing Grace...again: this is a guess but it makes some sense...I'm sure that the pipers in the group will have the real answer.

Best

AA