That dealership probably just doesn't know how to sell it. When I bought mine I had to have the local dealer order one in because they had decided they weren't even going to get any Africa Twins for their inventory.
I think in 2017 a lot of people were waiting for the ATAS which had already been announced for 2018. I bought in feb 2019 a brand new late 2017 model for a great price. It's the red one which wasn't as popular as the rally colours so maybe that's why it didn't sell before. Im so happy with the bike, no regrets and the red has grown on me more and more ever since, so much so that I wouldn't swap it even for a brand new ATAS.
Also, some dealers just ordered way too many bikes. If you do buy it, negotiate hard and you'll get a great price, plus if not yet registered you'll still have the full warranty
I doubt that a single dealership will have had a bike sitting in their showroom for this amount of time. its more likely that there has been some wheeling and dealing between dealerships and the bike has been moved from one place to another. another dealer who thinks they have a better chance of selling the bike - and as if by magic. along comes YOU.
I don't think its hung around for any specific reason. local circumstances count for a lot. And.. aside from the news of the new Adventure Sports model. There was also news of the changes to the electrics, which I for once considered a big deal and the main reason I didn't really look seriously at buying one until 2018... and then waited until the end of the year to actually buy. I was in no rush and i was fairly sure that Honda would be offering big discounts. and that is exactly what happened.
A few months ago, April, my local dealership got in three unregistered red 2017 bikes and sold all three within days.
My wife has 2017. Not a single issue whatsoever with it in 2 years. I guess the 2016's allegedly had some minor issues that were tweaked with the 2017's, but I have had zero issues with my 2016 as well.
My guess is that bike was moved from another smaller dealer or one that sells less of the Adventure bikes, to your deal for sale. But they will probably tell you if you ask. Also you didn't mention what color it was. From what I understand from talking to my dealership when they commented on my wife Candy Apple Red AT, the silver sold very poorly and Surprisingly the Candy Apple Red, not much better. I actually like her Candy Apple Red better than my Red/White/Black.
My Red/White/Black was a holdover 2016 that I purchased in August 2017. It wasn't hanging around as long as that one and was a demo (329 miles on it), but I got it on a "managers special" at a much-reduced price. Hopefully, you can ride away with it at a steal?