First of all it needs to be said that today Ethiopia has one of the world's fastest growing GDP, even with the present drought

It is hard to overstate the damage Mengitsu's Derg regime did to Ethiopia. During the 80's he actually told ministers to shut up about the drought because it was making the country look weak and impotent. He also did not assist western aid efforts and even devoted resources to fighting rebels instead of alleviating the famine.

Also Ethiopia's economic and cultural paradigms were shattered. True the monarchy did not work to raise living standards and governed through a byzantine patchwork of customs and laws, the Derg ruthlessly tore down any semblance of the old order. Many people lost everything and the nations commerce and industry was nationalized, often to be doled out to Mengitsu's cronies. One of his close associates (i think his wife or mistress) had a monopoly on school uniforms and she increased the prices exorbitantly. The students and their teachers protested. Mengitsu savagely cracked down on the protests, some of the students were taken to torture cells, where Mengitsu himself was purportedly seen joining in the torture.

Also when Eritrea gained independence, Ethiopia was left landlocked with a bitter rival contesting its new borders.

My source is "The Fate of Africa" by Martin Meredith

Before looking at the specific example of Ethiopia, it's important to note that the question of why some countries are more economically successful than others is EXTREMELY controversial and complicated. There's many many elements that contribute, including geographical, historical, and cultural factors. In popular discourse, it's common to blame Africa's current plight solely on colonialism, but this is a gross simplification. Indeed, as the example of Ethiopia and Liberia shows, those countries in Africa that were not colonized ended up just as poor, if not more so than others. Meanwhile, many countries in Asia that were also colonized have become much more economically successful than sub-saharan Africa. Again, it's a very complicated situation, but this makes it all the more important to look past simplistic explanations like "It was colonialism", or "It was cold war politics", as all of these factors have contributed to keeping a large portion of the African population in a state of destitution.

That being said there's a couple factors we can look at in the Ethiopian case. This includes, the difficulty of holding together the diverse ethnicities, religions, and linguistic groups that make up modern Ethiopia, which has at times caused civil war and war with neighbouring states (i.e. Somalia and Eritrea). There is also the fact of Ethiopia being landlocked after the independence of Eritrea. Furthermore, Ethiopia suffers from irregular rainfall, which has caused regular droughts, the most devastating of which took place in the 1980s (which was compounded by civil conflict and corrupt leadership).

However, the factor that I would most point to, and indeed I would point to for much of the current economic disorder of most African states, is a failure of leadership. As has been mentioned, Mengitsu's Derg regime was disastrous for Ethiopia. His divisive regime caused civil war. His megalomania prevented aid from reaching famine victims. His corruption prevented services from being carried out properly. In essence, he destroyed many of the institutions necessary to carry out a functioning nation. Politicians that have followed have been better, but have hardly been exemplary, and corruption continues to be a huge problem for Ethiopia. That being said, as others have noted, Ethiopia's economy is growing at a very fast pace today, so there remains some hope for the future.

Source: "The State of Africa" by Martin Meredith

Ok, there is a lot to talk about, so strap in for a long-ish post. At the outset, though, I want to push back about a statement you said about Africa as a whole.

why Africa is weaker economically than most of the rest of the world, and the general consensus seemed to be that it is the legacy of colonialism.

I think the older FAQ threads should probably be reviewed and updated. Yes, colonialism had an enormous impact on Africa, and it laid the basis for exploitative economic systems that enriched Europeans at the expense of local Africans. However, there have also been many factors in the years since 1960 that have also had tremendous influence on the economies and societies of African states.

Strongman regimes that enriched the powerful and politically connected created much corruption, and resulted in billions of dollars deposited in tax havens overseas.

Civil wars, and rebel groups armed by cold war superpowers and funded by illegally harvested resources created instability that slowed growth, scared international investors away, interrupted education and harmed infrastructure.

The AIDS epidemic has been keenly felt on the continent, and creates economic harm through the death of workers, leaves families without breadwinners, and the medical costs borne by families or the state.

Many states have relied on loans from the international community, particularly the IMF, to partially fund their budgets. Throughout the 1970s and 1980s, the trend was overall one of increasing debt to the IMF. In the late 1980s and early 1990s, many states had debt that were larger than their annual GDP. To ensure loan forgiveness, and access to future loans, many nations including Nigeria, were forced to cut government spending. Food and fuel subsidies were frequent targets of cuts, which overwhelmingly harmed the poorest.

Because of all of those factors, African countries have experienced a persistent problem with Brain Drain, where talented and fortunate individuals will pursue university education overseas in Europe or the United States because university programs in those states tend to be considered more prestigious than universities in Africa. After finishing their education, many of these scholars have looked at the political repression, instability, and economic malaise of their home countries, and choose to remain in England, France, the US, judging that they have better economic prospects or opportunity there. This has a negative impact on African economies, because they are deprived of highly skilled workers who could found or grow a company, conduct research, develop an invention, etc.

So, for the countries of Africa, colonialism has a role in explaining it, but it is not the only cause. They have wrestled with its legacy as well as many other challenges in the past half century.

(more to follow)

As a follow-up: is there reason to believe that widespread poverty was NOT present in medieval Ethiopia?