In Windows 10, Reader was officially merged into Edge. The app that remains in the Windows Store will gently encourage you to use Edge instead. This seems part of the direction shift from simple task-focused apps (back) towards branded monolithic apps as Microsoft tries to find a healthy middle ground between good for mobile and convenient for old desktop users. (It's been a bigger deal of public concern with the Messaging/Phone/Skype apps back and forth shifts, but that hasn't been the only place this has been happening, as seen here with Reader, Reading List, and Edge.)
