Not to be too reductive here, but I think a major part of the answer is "location, location, location".

The major states and cultural hubs of West Africa connected to each other across four distinct ecological zones (Saharan desert, the shifting edge of the Sahara aka "sahel", savannah and forest) in a network of trade and migration routes and from there across the Sahara to North Africa. With the arrival of European ships in the Atlantic, these networks densified further and pivoted as much to the coast as they did to trans-Saharan connections. The equatorial rain forest limited any possibility for further connections southward and the length of the sahel from west-to-east limited connections between the societies of the upper and middle Niger with the societies of northeast Africa, including the Horn of Africa. So that in some sense, western Africa was a distinct region.

Southern Africa's outward connection was to the Indian Ocean world via the southernmost Swahili cities and communities on the East African coast. Its larger states prior to the late 18th Century were formed by Bantu-speaking groups who arrived in the region somewhere between 300 BCE and 500 CE or so, compared to West Africa, where many of the major hubs of human settlement took recognizable shape after the drying of the Sahara around 2500 BCE, so that's also a major difference. In relative terms, the links between Southern African states and societies and larger global networks were weaker than in Western Africa in terms of volume, density and relative antiquity.

Axum and the other states of the Horn of Africa and the Ethiopian highlands in contrast had profound and multivalent connections to global trading and migration networks that were as old and as developed as Western Africa, if not more so, but these were primarily through the Red Sea and to the northern circuits of Indian Ocean trade. That meant many things: a different mix of languages than Western Africa, a different and earlier exposure to Christianity and Islam as well as to Mediterranean, Near Eastern and Mesopotamian societies in general, and so on. But also the Ethiopian highland was a very distinct environment with its own affordances and challenges, and in some ways it served as a kind of natural fortress or "focusing zone" that gave states in the area a distinctive institutional and cultural character--connected but also isolated all at once.