Pavane, I suspect it might have been social status. It was the fashionable thing to collect old songs and if you made them up yourself, it probably did the job without the work of chasing after sundry peasants with (frequently) disappointing results. Cunningham was considered by many to be "A man of letters", having started with a rather lower social standing and, of course, the same thing happened to Burns. One has to say that some of Cunningham's songs and poems were pretty good, going by what's available on the net.

BTW, as promised, here are:

References to Allan Cunningham in earlier Mudcat threads.

The thread on Scottish Emigrant Songs has a whole lot about Cunningham, including some information about Cunningham changing the song "My Ain Countrie" and adding two half-stanzas here.

This thread about Favourite Jacobite Songs refers to a forgery of "Wee Wee German Lairdie" by Cunningham. The same information is repeated by BruceO here and there is more about this song here.

Two posts in this thread state that the Scots songs in Cromek's 'Remains of Nithsdale and Galloway Song', 1810 are mostly fakes and here is some information about Cunningham faking songs in his own publication (Songs of Scotland).

That's all I've found.

Cheers!
Ian