Why can't all blu ray's have the same special features as their collectors sets?

[Movie shy]

I really want to get the 4 musical collection Blu Ray that includes: Singin in the Rain, The Band Wavon, Calamity Jane and Kiss Me Kate. I love musicals. I had been wanting to get Singin in the Rain on blu for a long time but held off because most of the special features on the collectors box set edition are not on the single blu of that movie. I love special features. I recently bought the new Sound of Music 50th anniversary blu just so I could have all the special features ( only had that movie on DVD never in blu ray). It seems that with the new 4 movie musical blu I mentioned Singin in the Rain is still missing most of the special features that are in the collectors set. Why can't all blu ray editions have the same special features. I do not need a big box set that just includes some nicknacks and is too big for my apartment space. I just care about the movie and the special features. I was very happy that The Sound of Music and The Wizard of Oz included all the special features if you didn't get the collectors big box sets. Anyone else feel this way?