I would imagine that people without smartphones are amongst those most likely to carry cash.
That was my thought.
Unless they've just accidentally run out of cash or just dropped their phone and broken it, the number of people fulfilling these criteria:
Does not carry cash
AND does not carry a smartphone
AND does carry cards
AND is on a long distance journey
...is somewhere around the 0 mark. If they weren't accepting cash, then that might affect more people.
If you add...
AND wants to buy stuff from an on-train buffet
...to that list then the number will be even smaller, as proportionally very few people on a given train actually do, which is why the economics of on-train sales are poor.