Husband and wife, grandma and grandpa, teenagers and even elementary school children are all sending e-mail messages and all want the privacy of their own personal accounts. While some Internet providers in major cities offer more than one e-mail address per account, that service is very rare in rural areas where competition is limited or nonexistent. Using World Wide Web browser software, like Netscape or Internet Explorer, an individual accesses a particular Web Page, enters a user ID and an access code, and their e-mail appears before them. They can also send mail from the same location. And the services are completely free.
Advertising seems to be the key reason.
These sites are electronic billboards and gateway sites for all sorts of commercial enterprises and products. In simple words, the more often you look at their page, the more likely you are to buy something. For me, that's no problem. These services are highly competitive and recently several of them merged together, creating just a little less choice. But there are still at least six major services and a whole bunch to minor ones as well.