These languages die when communities stop speaking them, often due to cultural assimilation, migration, or dominance of a more widely used language; in their case, it was all three, the natural homeland for Somalis is Northern Somali and modern-day SR.
Arbore, Dasanech, and Elmolo all split from Proto-Somali very early, around 1000 BCE, somewhere in northern Somalia, Af Somali has remained relatively untouched bar Arabic loan words which probably got incorporated alongside T Somalis, I can't see how a group of relatively territorial people would allow a foreign group to settle, and procreate unless said group brought a life changing technology that could allow trade and sustance to live in a changing climate, increasing the chances of survival, this creature is drought resistant, I remember reading a study on the Somali camel by the british and it could haul, and survive on less, majority of Somali camels are used for their milk, even during the heights of the trade.