Of those 6000 only 631 logged in to their account in the last month. Taking it further 413 in the last week.
Only 110 of this logged in during the last month with the number even less for the last week, 53 in total. Also South Africa Post has massive issues and they are often on strike, mail to and from there takes a long time.
2 logged in the last month, none in the last week.
36 in the last month logged in.
174 in the last month, 112 in the last week.
587 last month, 292 in the last week.
All this information can bee seen on the Explore/Search page
The other factor that will affect you is if the Post Offices are exchanging mail between those countries and yours. Up until recently Australia wasn’t exchanging with USA for example
Now to specifically answer your query from an Australian point of view, but does apply with a lot of other countries (northern and southern hemiphere) as well.
Population: Australia only has a population of 26 million, compare that to USA of 327 million.
Internet, many southern hemisphere countries have poor/slow costly internet.
Postage costs are high for a postcard to be sent internationally. $A3.40/$US2.45, compare that to your $US1.30
Time is a large factor, Before covid a postcard took on average 18-21 days to arrive in USA, since covid a lot of our mail has been going by sea, so can take months to arrive.
Of the other countries mentioned population is a large reason why there are not so many members in those countries.