dragonball mistakes

[Robojack]

It's because Piccolo was almost like a 'clone' or 'spawn' of the original evil half of Kami. Piccolo pretty much took the place of the original evil Kami, him being nearly identical to his father. That's why the good Kami didn't die, and also why in DBZ, when Piccolo dies in battle against Vegita, Kami dies along with him.

I hope that answers your question. I'm pretty sure that's the exact answer, otherwise you should just assume that Akira Toriyama wanted it that way.