That’s mainly due to the varying licensing agreements per region. It’s possible that a new show on Netflix will be available in the US but not in the UK, and vice-versa. This applies to other locations as well, which is why it’s possible for you to hear about new releases in other countries that are inaccessible in your location. 
It’s also possible that a title is available in your location and you download the TV show or movie there, but you traveled to another place where the title is not available. In that case, you’ll see a message that says: “This title is not available to watch in your area.”
According to Netflix, this notification means that Netflix doesn’t offer the content you’ve downloaded in the location you moved or traveled to. The title will only be available for you to play once you return to a region where the content is available. You have to connect to the internet and start the Netflix app so it can discover your location and when that’s settled, you can play the movie or TV show. 
If you want to know which Netflix movies are available in a particular region, visit this website.