Why do I receive more postcards from a certain country?

Probably because that country has many more active members (sending many more postcards) than other countries.

If you're curious, you can explore Postcrossing's members geographical distribution on this page: https://www.postcrossing.com/explore/countries

For a more balanced distribution of members and postcard origins, consider inviting some of your friends to the project. This way, a better balance can be achieved without being unfair to countries where Postcrossing is popular.