Over a year ago Microsoft officially informed that Windows Phone won't be supported anymore and company will now focus on creating apps for other mobile systems. In Europe all phones were withdrawn from sale some time ago, and today time has come to do the same in other areas. Some hours ago last units of phones with Windows Phone system disappeared from official Microsoft Store.
We don't know what the official reason for this is, but we strongly doubt that all these devices has been sold. A much more likely option is that Microsoft withdrawn these smartphones from their shop due to poor sales. If someone really wants to buy one of the devices with Windows Phone system, the only chance for it is to go to the physical Microsoft Stores, but over time getting these phones from any source will become impossible.