Why Public IP? Why no Private ?

Hi Friends, I am currently working on one ISP sort of project. I have a small confusion I hope someone could solve this. I want know that why ISP’s uses Public IP’s only why don’t they use Private. I know some small ISP’s who provide Internet Service over Cable net in India where they don’t need to use Public still they use Public IP.
I know having Public IP pool is very costly afair thats why I am confused.
Is there any limitation with Private?
Can’t I use Private IP’s for those who just want to access intenet and who never bother to explore their computer on internet still ISP’s gives Public IP to such users, why?