Ethiopia was a Christian country, and had been since the 4th century AD.
As one of the major justifications for imperialism was the bringing of civilization and salvation to the indigenous natives of African states through the introduction of Christianity, European countries could hardly apply these criteria to Ethiopia. The country was thus regarded as off limits by the major European powers.
Further,as Ethiopia practiced Orthodox Christianity, it had full diplomatic relations with Russia - any attack on Ethiopia would thus have caused tension with Russia. France and Russia were allied from 1892, Germany wasn't going to risk war with both over an African colony, and Britain was more concerned with Russian influence in central Asia and Persia than its limited influence in the Horn of Africa. All these factors combined to keep Ethiopia safe from attack by the Major European powers.
An attempt at conquest by Italy 1895-96 failed following a disastrous defeat at the battle of Adowa, 1 March 1896 (emperor Menelek II having recently received a large consignment of rifles and ammunition from Russia through Britain). Italy subsequently recognized the independence of the country under the terms of the Treaty of Addis Ababa, and Ethiopia formalized established borders with adjoining colonies of the other powers.