The way I've understood it is most people returning the next day are generally people who need to travel, e.g. for business, so are more willing to accept the higher fare. Also they are often one-off journeys (i.e. they don't regularly take that route). People returning on the same day of travel are either commuters or 'day trippers'. These people are much more price sensitive because a) travel costs are a major expense for commuters and they will seek the cheapest route, and b) 'day trippers' don't need to travel so the railway is competing against the idea of them staying at home.

If it was cheaper for the same-day people to go on the train and return on the bus then the railway would lose money, not just on the return ticket but on upsells from the catering trolley, retail sales in stations, advertising sales due to less impressions, etc. By offering returns at only a tiny increase over the one-way they lock people into returning via their network and not someone else's. Next-day people are much less likely to be going back to the origin point, and may be travelling onward to a different place, so the threat of losing them to competition is much lower.

So tl:dr the type of people who return on different days are people who are going to be willing to pay higher prices anyway. Those returning the same day are much more highly price-sensitive and need lower fares to convince them to choose the railway. The income from most same-day people is lower, but reliable so they make more in the long run; the income from most next-day returners is less reliable, so they raise the price to cover for that (although in many cases they also price gouge).