Abyssinian ethnic groups always had grudges against their neighbouring ethnic groups and have always wanted to rule over them for an obvious reason. Somalia have a little to do with this since they were far from the shores.
The Abyssinian expansion across the Horns of Africa began during the late 19th century when the Ottoman withdrew from the region and European colonial powers settling in.
This sparked a huge war in the region - Continious wars between Muslim ethnic groups, Abyssinia and the European colonialist, the Mahdist wars, the Mad Mullah revolt, Egyptian intervention and so on.
So, the question is how did Abyssinia expand and why wasn't Somali affected but the rest were?
First of all, Somalia and Somaliland was a colony and a British and Italian protectorate whereas the others weren't which is why they weren't affected. The idea of thinking Somalis were "stubborn" is laughable considering how their country were colonized or how Ogaden was occupied.
Anyway, the Abyssinains benefited greatly from this because the British chose them as their ally and supported their annexation of the South, West, North and East so they could establish 'Ethiopia' and the British respected and recognized their newly formed country.
They also received unlimited modern arms from the coastal regions in the North of Somalia whereas the local ethnic Sultanates had sticks and had no support. Their lands were in shambles and were isolated unlike in the past where they had regional allies (local Sultanates supported each other to ensure Abyssinia wasn't a threat) and had the support of the World Powers support (Ottomans and Egypt who viewed the region as a vassal state).
So, they stood no chance against Abyssinia. Italy managed to blackmail the locals in Eritrea into thinking how Menelik were going to invade their lands and how Italy would protect them as they no longer had a sultanate after the Mahdist defeat so they came in with recieving little to no resistance and were seen as favourable over Menelik's army so they reluctanctly accepted Italian colonialism.
Another thing is the British, France and Italy never wanted Afar, Oromo, Beja, Saho, Aderi or other ethnic groups to rule and viewed them as a hostile groups because of the religious difference, their historic wars against them and their ally (Abyssinia) which is why they divided the Beja and Tigre between Sudan and Eritrea, Afar and Somalis from the three countries, Saho and Jeberti from Eritrea and Ethiopia and so on to ensure they won't unite in the future and keep them subjugated by modern leaders who are seen as foreign to them.
I remember reading a book saying they would lose their colonial status if they were ever to rule the region again.
Historically, they have always favoured the Amharans and Tigrayans over them.