That's because the US military required it. Electronics and component reliability was so terrible that they started naming and shaming by publishing reports abo

That's because the US military required it. Electronics and component reliability was so terrible that they started naming and shaming by publishing reports about exactly how each part was total crap. It showed industry that everyone else was making garbage, and that the military would pay for quality. Add in the second source rule, and suddenly everyone is in the business of making better quality parts. Nowadays, there are additional market forces that drive higher quality. Technology also advanced beyond what you could do with discrete components and simple building block ICs. So the cost-benefit scales tipped towards allowing single source.