It’s a very complex system with 800,000+ members, not a simple one & the algorithm does it best to:
" What Postcrossing does is to try to maximize the number of different countries you are sending postcards to at any given moment. However, at some point, repetitions must still occur, in order to keep the system balanced.
Postcrossing allows repetitions to happen more frequently to members who are sending larger amounts of postcards at the same time, while being less common for those who can send less. Since those who can send more have (by consequence) a wider variety of different countries than the rest, we feel this is fairer to all.
If Postcrossing prevented any repetitions from happening, members in very active countries would be unable to receive the amount of postcards they were due — they would be sending, but not receiving! So repetitions are a requirement to keep the number of postcards sent and received in each country as balanced as possible.
Lastly, note that the amount of repetitions can and does change over time. Postcrossing dynamically adapts to the balance of each country at any given moment, and this is why you sometimes might see more or less repetitions."
https://www.postcrossing.com/help/why-do-i-have-so-many-cards-travelling-to-the-same-country
If you look at the math of the large numbers of members in about 18 countries compared to the rest of the countries, this balancing system, while not perfect, is essential to making Postcrossing work. It would not be a balanced system at all in a first in line system.
https://www.postcrossing.com/explore/countries
Some people choose the repeated country option to help the system out or choose to send cards within their countries as well and sometimes for different reasons. I know a bunch of Canadians choose to send more postcards internally in Canada because of the price of international stamps which are 3X the price of a domestic stamp for example. Both of those options are choices now & I think that’s a fair way to do it.
I focus mostly on making connections with people & I am patient, seeing that over time the system does let me send to more countries.