I can totally understand why “pristine” and “undisturbed” work, but why not picturesque? All three words are “positive” and indicate some form of “beauty”, or “perfection” or some characteristic of that sort that is typically associated with nature (vegetation, rocks, monadnocks, mountains).
Hello. The key is to focus on what is being described. In this case, it is vegetation. i.e. plants (not the large rock masses). The sentence tells us that the vegetation has been decimated (destroyed) by hiking traffic. (hikers have trampled on them).
Therefore the vegetation/plants/grasses are no longer in their ''pure" or “original” state.
Pristine = in its original condition; unspoilt. Like when you move into a new apartment before your kids begin to draw graffiti on the white walls. 
Undisturbed means essentially the same thing in this context, say, like the flora and fauna of the Amazon rain forest.
As you said, Picturesque indicates some form of “beauty”. Like the view on a hill a castle, rock etc. Even though it is positive it doesn’t mean the same thing as the other two answers, considering the context.
I hope that explains it a bit.