Getting a paper travelcard from a newsagent or as TfL officially calls them 'Oyster Ticket Stops' can be a hassle. Since the introduction of the portable Oyster selling machines, they've been unable to print out paper tickets, so the paper tickets they do sell (only One Day Travelcards) they have to get the date stamp and ink out before selling it to you.

Most of these retailers would rather you had your trusty Oyster Card and slap a few quid on that instead of purchasing the paper tickets.