While I agree there's likely a correlation to the two criteria you mention, there's a third which is think is more at the core.
That is that for a ton of ISP subscribers the concept that email doesn't need to be coupled with their internet service is not something they understand. For these people the idea that they'll need to change their email address when they switch ISPs is taken as a given. Just as if they were to move they would need to update their mailing address.
That is that for a ton of ISP subscribers the concept that email doesn't need to be coupled with their internet service is not something they understand. For these people the idea that they'll need to change their email address when they switch ISPs is taken as a given. Just as if they were to move they would need to update their mailing address.