Because you uhh, you have to make the choice? Even though God, if the being is all knowing, and all powerful, could set up conditions for every human being that ever existed to come to choose tobelieve in him, should he actually desire that.
So either, he doesn't desire it, can't do it, or doesn't know.
If he doesn't desire it, it doesn't matter. If he can't do it or doesn't know, then he is not God, and it doesn't matter.
I think it's because he's really needy and wants you to choose to worship him on your own. Never mind that you're being threatened with eternal torment if you don't, you choo-choo-choose him!