Why can't all special features from a DVD be ported over to the blu ray release?

[Movie shy]

I really hate when a blu ray is released but it is missing most of the special features from the DVD that had been released prior to the blu ray coming out. I also don't like when 2 of the same blu ray will be released at different times and not contain the special features that the first blu ray release had. Case in point is Guess Who is Coming to Dinnner. It was originally released on Twilight Time with an Isolated score and an Audio Commentary track and neither are included on the new blu ray digipack release. I like Digipacks but don't want to buy it for that movie because of the missing special features. I love special features and don't understand why all special features can't just be ported over to the blu ray being released (especially since most of the time the special features are in 480i).