Why were these territories left out by the European powers?
That is not exactly what the Berlin Conference was.
The conference did a few things:
Established a legal framework of "effective occupation" to determine legal recognition of existing (before 1885) and future (after 1885) territorial claims. Effective occupation meant that a European power or a chartered company had to 1) make treaties with African chiefdoms/kingdoms in the territory claimed. 2) build infrastructure like trading posts, mission stations, roads, ports. 3) have a government and administrators "on the ground" in the territory claimed.
Signatories agreed to allow free flow of trade and vessels on the Niger river, Congo river, and Lake Malawi.
the Conference recognized the specific claims of the International Congo Society to the Congo basin. the Congo Free State was established during this conference.
Signatories all committed to work to end enslavement and the slave trade by African societies on the continent.
The Berlin Conference in 1884-85 was not about drawing lines on a map dividing the continent up. The scramble to make territorial claims had already started circa 1870, and the division of the continent into colonial territories and suppression of African resistance would be an ongoing process until ~1905-1910. The Berlin Conference merely established a uniform "rules of the road" to sort out competing claims, as well as signaling that multiple European powers + Ottomans + the United States were interested in territorial claims in Africa.
Anyway, to address your underlying question about why Ethiopia and Liberia weren't colonized during the Scramble for Africa.
There were a few close-calls for Ethiopia. In 1868 Britain sent a military expedition to attack Emperor Tewodros II and free British envoys that were being held hostage. The British column secured the cooperation of Ras Kasa, the ruler of Tigray and rival of Tewodros II. Kasa allowed the British troops to pass through Tigray on the way to attack Tewodros' capital at Maqdala.
Under siege at Maqdala, Tewodros II committed suicide rather than surrender. The British looted his capitol, freed the hostages, and quickly withdrew from Ethiopia.
While Kasa had allowed passage, he was eager for British to leave, and relations between the British and Kasa didn't develop further. Eventually Kasa would take power as Emperor Yohannes IV from 1871-1889.
Later on, in 1895 the Italian government signed a treaty establishing a border between Italy's colony of Eritrea and Ethiopia. The treaty also established mutual diplomatic recognition between Italy and Ethiopia. Italy inserted a provision in the Italian version of the treaty that said Ethiopia agreed to become a protectorate of Italy. In the Amharic version that passage only said when Ethiopia had diplomatic dealings with other foreign powers, Ethiopia could consult with Italy if they wished.
A diplomatic crisis ensued when Emperor Menelik II denounced the duplicitous treaty, and Italy mobilized an army. Menelik mobilized his army, and famously defeated the Italians at the battle of Adwa. This thread by /u/Quadetvincet and /u/Khosikulu goes into greater depth about that campaign. Khosi points out that the defeat at Adwa was extremely shocking to the Italian government, resulting in the fall of the Crispi government, ensuring a second attempt to invade wouldn't happen immediately.
The popular perception is that Adwa is the battle that guaranteed Ethiopian independence. It is true that Menelik II managed to gain a great deal of international recognition for Ethiopia, solidifying it's independence.
However, Adom Getachew in Worldmaking After Empire points out that even after Ethiopia joined the League of Nations in 1923, European colonial powers like France and Great Britain expressed great concern over persistence of the slave trade in Ethiopia, and doubting the Ethiopian government's ability to exert control within its borders. Britain, France, and Italy were also concerned that Ethiopia continued to purchase and distribute small arms, contravening a treaty that those European powers had made to limit the importation of firearms into East Africa. Into the late 1920s, Britain and France were proposing that the League should impose European government advisers to help guide the Ethiopian government (which Getachew states is effectively advocating for colonization).
When Italy invaded Ethiopia in 1935, Benito Mussolini and his government were justifying their actions to Britain, France, and the US by pointing to the persistence of slavery in Ethiopia, and arguing Ethiopian government was incapable of putting an end to the slave trade.
Italy occupied Ethiopia from 1935-1941, when British armies and Ethiopian Patriotic Army drove out the occupying army. In Revolutionary Ethiopia, Edmond Keller argues that in the period from 1941-1948, Ethiopia was very reliant on British financial and technical support for reconstruction. In that period, it looked very possible that Ethiopia could end up as a British protectorate. However, the American military saw Ethiopia as a place to build a node in a global radio communications network, linking NATO with Western forces and allies in Iran, Iraq, Pakistan, etc. The inflow of American military and development aid from 1948 onwards ensured that Ethiopia did not end up as a British protectorate.
As for Liberia, I'm not going to go into quite as much detail. The short answer is that while it was never considered a true colony or protectorate, the United States did consider Liberia within the American "sphere of influence". So, when France annexed inland Liberian territory to French Guinea colony, and Britain threatened to do the same, the US exerted diplomatic pressure to ensure Liberias independence within the reduced borders.
This American belief that Liberia was within their sphere of influence also led to American corporate investment in Liberia. Famously, the Firestone corporation of Ohio controlled enormous rubber plantations in Liberia.
And like Ethiopia, the United States viewed Liberia as a strategic military partner. Early in World War 2, the US developed airfields in Liberia as bases for anti-submarine air patrols in the Atlantic.
TL;DR- Ethiopia's independence was a tenuous thing. It was a combination of military success at Adwa, shrewd diplomatic maneuvering, and luck that preserved Ethiopian independence.
Liberia's independence was preserved by American diplomatic intervention. But, the United States and American corporations treated Liberia as something of a quasi-colony.