Friend of mine left his 1150gs in my garage many years ago when he went on holiday. Said I could use it but was big into sportsbikes at the time (I had a Fireblade) Before he came home I decided to give his bike a run. Yes, it was totally different to the Fireblade but I just thought wow, vibraty (new word), heavy but comfy and I could see over cars and hedges.
So I guess it was that first ride on the 1150 that made me realise I liked that style of bike and it suited my kind of riding.
That ability to sit up high, comfy and style of bike and riding does not really explain why one bike has driven the whole boom. I have sat on a GS, but not ridden one. My reasons for not getting one and going for a KLE500 and then a Versys was those advantages of the GS, without the weight and cost to buy.