I had a Tandy TRS-80 computer, followed by a Commodore 64 in the early 1980s. I set up a website in 1993, and had been running a bulletin board since the early 1990s.
It clearly wasn't as common as now, but computers were around for people born in the 1970s. It's just that in most schools, you had a computer room and didn't have every class with them, or people bringing their own laptops.
...but getting back on topic, the likely reason e-tickets are so popular is because people have been used to getting barcode type tickets for many, many years now (well over ten years) for events and other forms of travel, like flying, so the railway hasn't introduced some fancy, confusing and complicated system - it has simply offered what people are used to.