Viewdata is a Teletext style information retrieval implementation. Its development pre-dates Teletext's and runs on a phone line, much like the Internet, as opposed to Teletext which actually runs via televisions.
Originally the format required purpose built terminals but the medium has now, much like teletext in that respect, found its way to the Internet.
Thus the medium survives. Initially telephones would be used to navigate but the Internet and its associated input devices have superseded this.