There are 2 possible reasons:
1. You do not have enough stars.
Open the world tour window and find a country with the lock on it, hover the cursor over it and you will see how many stars are required to unlock this country, your current amount of stars is shown at the top of the window.
1 - Your current amount of stars. 2 - Country that has mouse hovered over it. 3 - Information about the required amount of stars.
2. You have gone through all the available countries. But don’t worry, we add new countries regularly.