The message from Alasdair contains these words:
It's almost certain to be covered by covenant, where it is on common grazing. Besides, a lot of the peat on Lewis is quite shallow. Our peat-bank was about five feet thick, so the good black peat was close to the subsoil. At another location the bank was only three feet deep and there was very little black peat at all.
Another thing to bear in mind is that Lewis (and presumably, many other islands) is littered with rocks and would make large-scale extraction financially unattractive.
Remember, there has only been a comparitively short time for a build-up of peat: virtually the whole of Lewis and Harris was forested until Eric the Red burnt it. (Because the best and strongest galleys were made there, and harried and hampered his expeditions.) Lewismen never bothered to replant - surprise-surprise!
We often unearthed (unpeated?) charred tree stumps while digging.