It's because the countries are named after the people rather than the people being named after the country. So Ĉino would be a Chinese person and Japano would be a Japanese person, just like Ruso and Germano are a Russian and a German whereas Rusio and Germanio are Russia and Germany. For country based names we replace -o with -ano to refer to people. Brazil is Brazilo and a Brazilian is Brazilano.

Basically, if the people were named first and the country took its name from them, the -o ending is the people. Yet if the country was named first and then people took their name from it, the -o ending is the country. Whichever came first, gets the -o.

Correct me if I am wrong, countries end in -io, languages in -a, and people/nationality in -o

Antarkto apparently, but I would have preferred Malarkto