Generally speaking, the cornered the market through building good products that beat the competition. Do you remember free email providers prior to gmail? ISPs who you maintained a paying customer relationship with offered worse quotas and outdated web mails.

Yes, I remember plenty of free providers (as well as quality paid ones, and ISP ones, for example GMX and Ilse), including with IMAP/POP3, TLS/SSL support, anti spam features, anti virus features etc. Gmail was unique in three ways: it had a huge quota (for that time), and it offered a slick web interface. The other way Gmail was unique was the invite. If you gotta get invited to become part of the (secret) club, that makes it more special.

I remember postmaster.co.uk and hotmail.com, they were around way before Gmail, and were less invasive, but I got my own domain and I serve my own emails since 2000 so I got that covered. I like my emails like I like my newspaper: NOT to read them together with someone else. I do know that emails are clear text etc but it's not fun that I get an email about XYZ and then every website I visit shows me ads about XYZ.

I remember Hotmail