Countries may be rated by many various categories, such as how expensive or interesting they are to foreign tourists. They can also be rated by their openness and friendliness toward visitors.
You might be shocked to find out that, for example, the USA is number 102 on the list of the least welcoming places, 140 being the most unfriendly land.
Mongolia used to be a communist country. It was closed for quite some time and opened up to foreigners only 20 years ago. That's probably the major reason why it is not yet experienced enough in hosting tourists. The country is beautiful, with huge spacious areas of land that are low populated. There local people are not yet used to being visited by foreign guests.