Why did Ethiopia become increasingly isolated from its neighbors over the centuries?

Mainly because it is unique culturally. Ethiopia is surrounded by countries and cultures that have been converted to Islam. Ethiopia is the only predominantly Christian country left in the region. Secondary reasons include the cold war where Ethiopia was more on the Soviet side. Another might be that it was the only country in the region that was never colonized by European powers (Italy's brief occupation is an exception). So they have followed their own path without any major influences from outside.