I will leave my vision on what has been done wrong in my country, Brazil.
Many people had a Windows phone here back in 2014, so we were a key market to MS's mobile division.
When they decided to switch from WP 8.1 to the W10M, all they needed here was offering new models.
The problem was: the vast majority of the Windows Phones here were mid and low end. The higher end models used to sell great too, but they never competed with top droids and the iPhones. So the first mistake was to announce only the 950 duo. Then things got uglier. After placing local webpages for the products with a 'coming soon' label on them, the official launch never happened here. These brand new W10M devices never got officially sold on our country. And so did any other W10M device.
With no new device on sale, it's gotten more and more difficult to spot a person with a Windows phone. So developers stopped releasing new versions of their apps. Banks' apps stopped working properly, third party apps for services like YouTube needed weeks just to return to a normal state after a slight change on Google's side.
It's simple as that. No matter what MS did and does in the development front: with no new devices, these efforts are simply useless.
Then there's the problem of switching from a perfectly stable version of an OS, WP 8.1, to a crap of a beta software called Windows 10 Mobile. It took MS more than a year to make things look like a finish product and to bring the OS as a whole to be almost as fast and stable as the old 8.1 was (and they still haven't!).
Let's be honest: the 950 duo has great HW. They've listened to us customers: phenomenal camera, removable battery, dual-SIM, expandable memory, great screen, great battery, Iris scanner. It's all there! But they killed it on the software front and on the limited offer.
I find more and more difficult saying goodbye to this incredible piece of HW, but I got to face the truth, it's really dead by now.