Ask HN: How did Digicert come to dominate valuable PKI certificates?

I have noticed that many corporate tech companies rely on Digicert, such as the fact that it is the root certificate for the consumer websites of Microsoft, iCloud, Facebook, Amazon, banks, even HN. This is despite the fact that browsers don't even discriminate in value between different PKI certificate providers. It is much more obvious why Lets Encrypt came to dominate the world of open-source projects (it is free). But why did so many big players centralize around Digicert? Is it just human behavior, copying?