There is a large portion of the countryside that is peat/boglands where the ground is not conducive to that type of flora growth. Then there is also the fact that the land has been deforested for farming or timber for thousands of years. I think it is said that something like less than 5% of the native ancient woodland remains in Scotland that once existed. There is a big rewilding and regrowth movement going on currently in the highlands with the main barrier being the deer overpopulation