Gmail basically took over email through centralization. A lot of companies and people use it because the price is fair and the spam filter works better than almost anything else.

I have tried FastMail and I like the company, but the spam filtering is not good. But good luck even trying to replicate FastMail anti-spam if you roll your entire email stack yourself. I have run my own email server even well before Gmail was popular and it was a nightmare, I'll never do it again. If a company asks me to do it, I'll quit.

Even if you put Gmail aside, email became reputation based, which is naturally going to mean the larger centralized platforms will succeed over a few independent outliers because they will always have a better reputation and control what is allowed to go out or in. Just like how the United States still has a large role in the internet and what is seen internationally because so much is still centralized within those borders, or the centralized companies that are within those borders control it.

Which was enabled in large part from their centralization of email.

Gmail's spam filter is one of the worst. About once a month some gmail user tells me that he found my mail in the spam folder. Of course it is easy to keep down spam when you don't care about false positives.