In reply to Phil1919:
A bit more complicated than that. You can also add sheep farming, grouse shooting, and a wildly unequal land ownership that allows eco-damaging field sports-led land management practises to cover absolutely vast areas: not so much a patchwork of land uses as a monopoly/monoculture. In addition some climate changes have occurred since Scotland was heavily naturally forested (it's wetter and cooler now).
All that said, poorly controlled deer populations are the big one. Only where deer numbers are more sensitively (ie very tightly) managed or deer are fenced out does woodland regeneration stand a chance.
I would say our current tolerance for the excesses of the stalking and grouse shooting industries represents a huge national failure. Scottish Government should have got to grips with it years ago, and if we're serious about increasing biodiversity, restoring degraded habitats and mitigating climate change then it's got to happen.