1 Due to music licensing
In order for Spotify to launch in new countries, it must acquire the license for the music it will play in these countries, which takes time and isn't always an easy thing to do.
2 It's still expanding
Spotify is working on launching in new countries one step at a time.
3 The country is not a valuable market
Some countries are not considered a valuable market for Spotify to launch in which may be due to political unrest, civil wars or religious reasons.
4 It won't bring in money
Spotify has to consider how profitable it would be in the countries it wants to launch in terms of whether people would pay for ads or become premium users. (See Why Album Sales are Down?)
5 People don't like to pay for music
Some countries consider piracy the main source of downloading and listening to music, which means Spotify might not make money in them.
6 They're developing countries
Developing countries and war-torn countries are not a target for Spotify since music may not be popular or considered important in them. (See Why is Egypt a developing country?)
7 Weak internet connectivity
Countries with weak internet connectivity will cause problems for Spotify and its users, which could be why Spotify doesn't launch in them.