Will they ever be available? I see that at least Dragon Ball is on Amazon video streaming, but only in SD.
I own some of the blu-rays, but it would be nice to have them in digital format instead!
Any question of "Why hasn't X happened?" tends to be tricky to answer, because there isn't really much information on it unless there were already plans to do something (it's like asking "Why hasn't my favourite manga/light novel/video game/ukiyo-e been turned into an anime?" - the answer is just that no-one's bothered doing it yet and there's no real way of knowing if anyone even tried).
That said, here are some reasons that are mostly speculative:
I'm sure there are more reasons, but hopefully those are enough to go on for now.