I seem to recall being asked to take surveys after buying Johnston Press £10 tickets in the past so perhaps the market research has shown that there's a body of early retired people who find this kind of deal attractive.
While it's galling that the rail industry yet again takes 30-55 year olds for granted, I can see that opening it to all comers might create some unattractive business in terms of stag and hen parties and groups of sports fans travelling together. By definition, most of my age group's leisure travel (I'm 46) is done either at weekends or when on annual leave, in many cases using the same season ticket as for work.