Because it wasn’t done right.
I know that doesn’t help, but you haven’t given us enough information on what tools you used.
Using free tools, you should start with DVDFab HD Decrypter to break encryption and get the movie onto the hard drive. If it is small enough to fit a dvdr without compression you can burn straight to a disk with ImgBurn. If not, then use DVDShrink to reduce it in size before burning.