If I were to invoke haiku for this, I'd say that it is because it is. And that's about the whole story. I don't know how old you think Kingairs are, but the fact is that its' pressurisation system was, like the aeroplane itself, not made yesterday.
Personally, I've never had a problem with the functionality of the system and really fail to understand why this seems such a problem. I set the pressurisation control to where I want it and monitor cabin climb or descent, and the differential, to make sure everything happens the way it should. Once I've set it, the system actually IS automatic in that it will take the cabin to the desired cabin altitude and maintain it for me.
All I have to do is watch to make sure it happens. And, of course, I need to know what to do about it if it doesn't happen.