Presumably that Stirling isn't significant enough to make CityLink's list of "Six Cities" (but does make ScotRail's "Seven Cities")
The way that CityLink has evolved over the years is a good example of how travel in Scotland has changed - back in my day a Glasgow to Aberdeen service would generally stop in central Stirling/ Perth/ Dundee (and some even at Arbroath, Montrose etc) - just like an Edinburgh - Aberdeen service would go into central Dunfermline/ Kinross - but they've slowly given up on such stops, the Aberdeen/ Inverness markets are more important than the cost of running into and out of intermediate places - so now the "local" stuff is in the hands of Stagecoach's separate "express" services. Partly due to the way that the road network has been improved (bypasses, motorways), partly due to the way that the train services have upped their game (from the 1980s onwards, with the start of the ScotRail brand, but various subsequent improvement to speed journeys up, e.g. many Aberdeen services running non stop from Leuchars to Haymarket - I'm old enough to remember some Aberdeen trains stopping at the likes of Kinghorn in the days when BR saved a couple of quid by merging the "local" and "long distance" services at times.
Thirty years ago, Aberdeen/ Inverness were a long way from the Central Belt - single carriageway roads and what not - nowadays, people wouldn't be put off by a day tip to Inverness/ Dingwall for the football
At the same time, places like Stranraer feel comparatively further away (from the Central Belt), given the lack of similar improvements