Why do some Blu-rays have very few special features?

[ReduxInflux]

with what few blu-rays ive checked out, i noticed that special features, unless 2 discs are involved, aren't standard (and not that they're incredibly important anyway). but i'm wondering why, with 50gb of space, aren't bd's packed with extra featureage? does the hd film itself - in all its brilliance - really take that much space? can someone explain to this tech ignoramus why this is? gracias.