There are some reasons why it's not so easy to just offer Premium in any country.
1. There need to be agreements between Spotify and the music labels about playing content in that country.
2. Spotify doesn't offer Premium in a country where Spotify hasn't built up a support channel for it.
Here's a quote of the FAQs about unavailable countries:
"Spotify is launching regularly in countries around the world. Please keep an eye on the Spotify blog or signup here to be first to hear. We hope to see you soon! "