Board Thread:Manga Discussion/@comment-71.52.75.8-20130914192419/@comment-26432938-20150619225632

Okay so I'm super late but anyway, this is my theory and it's really simple:

When one gives immortal blood to a mortal, the mortal turns into an immortal.

Since Dio was dead when Jotaro decided to give his blood to Joseph, Dio was no longer immortal so his blood couldn't make Joseph immortal.

That's all.