Routes for tours are chosen based on:
1) Parcours
2) Money stumped up by starting and finishing towns
3) Remoteness for spectators
4) Overnight transfers
5) Likely weather
Parcours for Scotland of course is potentially great, best in UK
Money is an issue as soon as you get away from the Central Belt/Border towns
Again, Highlands are just too remote for a decent number of spectators
Transfers are a big issue in a stage race with no rest days. You could fly everyone from Aberdeen to wherever but logistically it is a big ask. This is the big one I think
Weather in Scotland in September is wetter and windier, on average, than other parts of UK
So unless ToB becomes a 2 week race with a rest day, I can't see Scottish Highlands featuring