Actually it depended mostly on hindsight bias. For those companies that succeeded, we invent a narrative and apply it backwards in time to decide that "core competencies" or "diversification" was the critical inflection point.

But it might have been due to the rise of the dollar against the yen, or the effect of an outbreak of gastro in the plant of a critical supplier in Guatemala, or because a supplier's supplier went bankrupt and your supplier is preoccupied trying to get money from the smoking ruins of a business you know nothing about.