Windows 10 Mobile was a very big project, which in opinion of Microsoft should have been milestone on the mobile market. In real Windows 10 Mobile was a failure, and Microsoft wasted a lot of money on this project. Even producers abandon their projects dedicated to this system, and Joe Belfiore, one of Microsoft employees confirmed that company won't release new version of Windows 10 Mobile.
Microsoft still will release updates which will fix bugs in system and will add some security updates, but company won't add any new features to the system. System hadn't received many good reviews, but in opinion of Belfiore the main disadvantage of Windows 10 Mobile was lack of apps projected for Microsoft's OS, which were available on other systems like Android or iOS. Definitely it was a problem, but Windows in mobile version had more troubles, and that's not surprising that Microsoft doesn't want to develop this project.