I think that's the largest factor. You started with Hotmail, Gmail, or iCloud depending on when you made your first account, or what phone you chose, and then..

I think that's the largest factor. You started with Hotmail, Gmail, or iCloud depending on when you made your first account, or what phone you chose, and then... why change? The UX is typically better, and that was especially true back when Gmail was taking over the market and introduced everyone to the concept of never needing to delete old mail. Good looking apps are available for every device, with minimal setup. Deliverability is so good most people have no concept of ending up in their contact's spam folder. It's what your friends and family are using, and its free. Concerns like privacy or being able to change provider without losing your address are abstract and out of sight.