What specific app was written for Win 10 that won't run on Win7 ? ?
Windows 8/Modern Apps will run on Windows 10, but not on Windows 7. (Basically stuff from the Windows Store)
Windows 10 App Framework apps won't run on Windows 7 either. (effectively the same deal)
I'm not aware of any standard desktop applications that can run on Windows 10 (or windows 8, for that matter) but not on Windows 7, but I wouldn't be surprised if there were programs that weren't
supported on Windows 7 simply because they were never tested on any system earlier than Windows 8. There are some changes to the desktop API but they are rather minor and I'd be surprised if any vendor decided to drop support for anything later than XP.