It was due to a kind of “gentlemen’s agreement” among the powers that be, that Ethiopia should never be colonized. During the Crusades, the Kingdom of Axum in Ethiopia sent warriors to fight along side the Crusaders, and this was never forgotten among the Europeans. in the 14th century, the King of Axum (then called the Zagwe dynasty) sent a delegation to Europe to meet with the Pope, and they traveled as far North as France. In the 16th century, Ethiopia was overrun by a muslim (most likely Oromo or Somali) called gragn mohammed (or mohammed the left-handed) who nearly destroyed all Christian churches and monasteries. The Ethiopian King sent a plea to the Pope, who called on the Portuguese (a growing power in the region) to come to the aid of the Ethiopians. The Portuguese had 2 centuries of influence in Ethiopia from that point on, but never conquered Ethiopia.
On the muslim side, there was the story of the first hijra, wherein mohammed and his companions were being persecuted by the Meccans, so they fled in various directions to avoid capture. One delegation went to the Kingdom of Axum in Ethiopia, where they met with King Armah to seek asylum. The muslims began by praising their love of Mary and Jesus (knowing the Christian view) and thus the King granted them asylum. In fact, one of the delegation was mohammed’s brother-in-law,…and he actually converted to Christianity and remained behind when the other muslims left to rejoin mohammed (mohammed had his wife married off in revenge). Because of this act of kindness on the part of the Ethiopians, mohammed ordered that no muslim should ever conquer Ethiopia (as mentioned above, that gave them about 900 years of freedom from forced conversions until gragn mohammed came along).
So, both the Europeans and the muslim Arabs respected this agreement about leaving Ethiopia alone, which is why it was never colonized. In the 19th century, Italy had just become a unified country (they call it the reunification, which is a misnomer, since it was never a unified country before). The new Italian Kingdom made gains in Africa by conquering territories from the dwindling ottoman Empire (the ottomans had cowardly destroyed the coastal city of Ottranto and massacred its inhabitants, so Italy had every right to any ottoman territory). As Ethiopia had a long-standing relationship with the Italians, the Ethiopian Emperor Menelik II gave what is now Eritrea to the Italian Kingdom in exchange for technology, agricultural equipment/labor and coastal protection (Ethiopia had never been a sea power, so they preferred to delegate this). As the story goes, the agreement read differently in Amharic as it did in Italian; the former saying Ethiopia was an ally of Italy and the latter saying it was a vassal state. Thus, the Italians began to encroach further and further into Ethiopia, thinking it was their right, culminating in the battle of Adawa…which the Italians lost.
The ramifications of the Italian loss cannot be understated; the Italian people did not want a war with Ethiopia (they blocked and tore up train tracks at stations carrying Italian soldiers to war so they couldn’t leave). When the news of the Italian defeat reached the people, the Republican government under the traitor, Francesco Crispi fell as a result, and Italy’s aspirations as a colonial power in Africa were ended. Relations between Italy and Ethiopia were normalized again shortly afterwards, and in fact many of the major building projects in Addis Ababa, such as the Cathedral and Palace were architected by Italian prisoners who then remained in Ethiopia.
40 years later under the Fascist regime of Benito Mussolini, Ethiopia was once again invaded. The fascists took the opportunity to take revenge on Ethiopia for the previous defeat, and committed several war crimes, such as the use of mustard gas on civilian populations. The Emperor, Haile Selassie had fled and sought asylum in England during the invasion. And while the Fascists did undertake many building and road projects during their occupation, this is not seen as a “colonization”, since it lasted less than 5 years and the government in exile always existed (those who argue it was a colonization, are simply racists, since no one argues that France was “colonized” by the Germans during their invasion).
