Geographically, northern hemisphere has more countries (being 40% land and 60% water ; while in the southern hemisphere it’s 20% land 80% water), which means more population in the north
Also maybe you can see it from economy perspective, northern hemisphere has more developed countries. Postcrossing isn’t a hobby for those who are still struggling with basic needs in many developing and underdeveloped countries
Maybe the culture of sending postcards, greetings cards, mails or just using postal service is dead / dying.
Majority of people I met online (even who live in northern hemisphere like Europe) will shrug and ask what postcrossing is. And when I explain, their reactions mostly are "Do people still do this?"
There are a lot of factors~
Indonesia lies just on the equator and majority of the population lives in Java Island (Southern hemisphere), hope an Indonesian postcrosser (from southern hemisphere part) will draw your address!
As for getting more people into postcrossing, I personally find it hard (in my own experience), because people aren’t that interested anymore to do correspondence like postcrossing/ snail mail. And majority who still do, usually have philatelist background as hobby and collector, while few started from snail mail nostalgia