Pick the one everyone else is using. Your friend has a Hotmail? You make a Hotmail. Everyone switched to Gmail? You'll also switch to Gmail. Also for a lot of people, email is just email. They don't even know that you can choose a different provider.
I picked the wrong one when I was a kid. I barely know why I picked the instance I did. It is hard to pick one. I say when trying to join lemmy, there should be interests tags that you select and an instance is populated for you.
You'd get an email address from your ISP. Early on you'd just dial the ISP, send/receive email, and then automatically hang up. College freshmen were assigned a school email address.
Eventually, "web mail" became popular because you could log in from any computer, like at the library.
By the time email became unavoidable, everyone had already been assigned at least one email address. It was seen as a major feature of the internet itself.
Well the concept of email is much more popular than the concept of a reddit style platform or even social media entirely. So information spread more easily.
And nowadays people just sign up for the account "required" by the OS of their phone which mostly comes with an email address.