There have been other threads on this before but broadly speaking the concept of a single standardized rectangular flag representing a country, its government, its people, its culture, and just everything about it in general was a European idea. It spread to the rest of the world by colonialism.

By around a hundred years ago most of the flagless states outside of Europe had been colonized and nearly every independent country had a national flag. When those colonies started gaining their independence a few decades later, it was natural for them to adopt all the standard trappings of statehood, including a national flag. The last outlier was Bhutan, which didn't see a need to have a flag until the signing ceremony for the Indo-Bhutanese Treaty of Friendship in 1949.

The only other country to have gone flagless since then was Guinea, which became independent quite suddenly in 1958 and didn't have time to design any of the standard national symbols. By then of course this was considered very unusual, and there were a lot of news stories about Guinea's independence basically took the angle of "New country doesn't even have a flag!" They adopted one a month later.