why countries have different store i.e different content ?

All content is licensed to Apple by the content providers/rights-holders on a country-by-country basis. Each country therefore has to have its own store and you need to be in a country to use its store. If a particular item isn't available to buy in your country then its either because your country doesn't allow it, or the rights-holder for your country hasn't granted Apple a license to sell it there.

If there is a particular item that you would like to buy then you can try requesting that it be added, but without the license Apple can't sell it there : http://www.apple.com/feedback/itunes.html