Well - a combination of factors to be honest.
I found that even post-updates, the low speed fueling led to very lurchy performance around town, and while the throttle was all kinds of fun to abuse when romping around, I felt it was actually very tiring to ride with for long distances*. I don't have space for a second bike, so I need something I can live with fairly easily as well as being entertaining.
At 13000 miles, the dashboard failed - replaced in warranty but would have been *very* expensive if out of warranty. The dealer was sadly unable to replicate the mileage or the service indicator intervals. Couple this with the fact that UK dealer servicing was *astronomically* expensive, even with the valve checks being every 15k, and I prefer to do a lot of mileage per year. (Also, being a US import, and a Ducati I was paying absurd insurance).
*Mine was a very very early 2010 model - I don't know if this has been addressed in later years.
I know the GS mechanically well, having self-serviced my last one entirely. I briefly pondered a K1300S - amazing motor, but looked at honestly how often I'd exploit it, and still keep my license. 0-80mph is the area I find the most fun - that bike really lit up when well into 3 digits, and I knew I'd be there too often if I rode it.
At some point I'll get something for weekend lunacy - I LOVE the Hypermotard, and the air-cooled versions are simple-ish to service. For the rest of the time the GS ticks all my boxes - hell, even did a few trackdays on my old one, and wasn't left behind by my sportbike-riding friends on the road much. Well, not at least until *well* into arrest-me-now territory.
Edit: Also, fuel mileage. I barely got terrible mpg on the Multistrada. Frequently found myself filling up at 140 miles or less - and I really hate stopping. I used Fuelly.com to track it for a while - I got LOW 30s.