There is NO rational reason for airbnb to choose to ignore 50 million potential customers, the cost is not that great. While a Windows 10 UWP would be great (allowing use on all form factors; phone, tablet, desktop, gamestation), there is still a large contingent of Windows 8.x users, so it might make best business sense to port their app for Windows Phone 8 (and the Windows 10 mobile customers could still utilize it). I know that all of my family members, all of my employees, and the 20 or so of my friends who use Windows phones would certainly appreciate support from airbnb.