Why DVD9 unable to play in Blu ray player?

DVD9 discs are formatted for standard DVD technology, which uses a different encoding and storage method than Blu-ray discs. Blu-ray Players are designed to read Blu-ray discs, which have a higher capacity and different file structure, but they are also backward compatible with standard DVDs. However, if a DVD9 is not playing, it could be due to compatibility issues, region coding, or the specific player not supporting certain formats or disc conditions.