In the early pioneering days of the commercialized Internet, email addresses were inextricably linked to your ISP. You paid for an ISP connection and you got an email box, with MTA and MUA service to match. You were reluctant to switch or leave your ISP, because that also meant leaving behind your email address. Of course, a minority of nerds got around this with their own domains, etc.
However, it seems that Google, AOL, Yahoo!, Hotmail, and other players got into providing free email services and eventually grew into giants that supplanted every other MTA service. This was not an accident and it was not merely our appetite for “FREE” but it was a very calculated plan by the industry. Those early ISPs did not have a business model that admitted monetizing our private data; they seemed to have a more respectful attitude for keeping it private. Perhaps that was a result of being telecommunications-based companies, rather than advertising or entertainment.
If a service like email provides such endless treasure troves of personal data, including a social graph and glimpses into our private daily lives, why not provide it for free and monetize opportunistically on the data itself? The free email services killed the paid services, not by being better or cheaper, but by being bigger, centralized, and more persistent. The main reason I signed up for Yahoo! was because it would be an utterly stable presence. I saw my parents and others so hopelessly attached to an ISP-based email, but I couldn't end up like that.
After streaks of losing my home and non-payment of bills and moving around over decades, the most stable point of contact for me has been a "free" email address.