"Cost" of switching
Many average folk will still likely use their provider's email service with the domain of their provider as opposed to web email - the "cost" of switching providers in terms of hassle to them, and to manage transitioning everyone and every organisation they deal with to a new email address is likely a barrier. All your utilities and banks etc. are not going to accept an email change by email! So I'd have to call them all up and wait for ages while they tell me that my call is really important.