I don't want to get into a slanging match with Always looking up but whilst it is true that "We have easy access to plenty of golf courses, castles and distilleries, as well as world renowned Royal Deeside and the Cairgorms." that's not how the rest of the UK, never mind the World - sees it. Looking at a map Aberdeen looks quite a ways from Cairngorm whereas Inverness looks much closer. then a lot of tourists want to do Edinburgh as well.
The other attractions aren't too much different from any other medium sized UK City - Dundee has a National (well,,, a branch of a National) Museum and ABZ? Not much TBH. Plus it's still an expensive place to stay or hire a car compared to other places in Scotland. It has no reputation as a foodie spot nor a clubbing center and I doubt you get many Hen parties heading there for the weekend.
It IS possible to turn a City round - Newcastle or Leeds anyone? - but it takes time. The best thing for Aberdeen would be to leverage the skills associated with the oil in things like business, IT and light engineering but I suspect that everyone takes the easy route when the oil price goes up and defaults back to working for the oil companies rather than looking for other business.......