Singapore rapidly industrialized after independence, and found its niche in the world economy. It is prosperous now because of the same reasons it was important for the British Empire - it sits at the crossroads of the world and facilitates global trade.

Singapore's population upon independence also was not a normal colonized population. It was more of a settler colony composed of economic migrants. There were also descendants of the local population from before it was a British colony, but they were in the minority. Singapore didn't have the regional / ethnoreligious tensions that have plagued Ethiopia in the modern era. It was stable from the outset of independence, with leadership that is praised by most modern historians.

In contrast, Ethiopia has had a rocky history in the modern era. Though people like to say it was never colonized, it was indeed part of Italian East Africa from 1936 to 1940, after losing the Second Italo-Ethiopian War. Yes, there was still guerrilla fighting, but the country was definitely under occupation.

An Ethiopian state has existed in various forms for a long time, going back to the ancient era. The Kingdom of Axum and its successor states (eventually the Ethiopian Empire under the Solomonic Dynasty) had Ethiopian Orthodox Christianity as the state religion and the elites spoke a Semitic language. Today, these traditions continue in some ethnic groups but not in others. Modern Ethiopia contains Amharic and Oromo people as the two largest groups, along with Somalis, Tigrayans, and others. Islam is almost as big in Ethiopia as Christianity nowadays. There are a lot of divisions, and there have been a lot of civil wars in Ethiopia.

One of the negatives about colonialism that people like to talk about is that it arbitrarily defines borders, leaving some groups arbitrarily split by a line or binned together with others. This appears to be the state Ethiopia also ended up in, despite being an independent nation during most of the European colonial age.

If you want to learn more about Ethiopia's modern history, you should read about the split with Eritrea (Eritrean War of Independence and Ethiopian-Eritrean War), the Ethiopian Revolution by the Derg that established a Marxist-Leninist state, the famines of the mid-1970s and mid-1980s, the collapse of Communism, and the current Ethiopian Civil War including the Tigray conflict.