Why do some industries naturally collapse into duopolies?

Looking at aircraft manufacturing, semiconductors, cloud computing, and even smartphones, some sectors seem to settle into a stable duopoly, often for decades.

In other industries, competition stays fragmented, even when the products appear simpler or less capital intensive.

I’m curious how HN thinks about this: – What specific conditions cause a market to converge into two dominant players?

1. Is it mostly capital intensity, switching costs, regulation, or something else? 2. And which industries today are showing early signs of heading toward a duopoly?

I’m trying to better understand the mechanics behind “natural” industry consolidation.