Northern started issuing them instead of cardboard tickets a few years ago and other TOCs followed along after Network Rail/Cubic finished adding barcode readers to ~all ticket barriers.
I understand why: paper barcode-only tickets are significantly cheaper than cardboard tickets (their lack of a magstripe allows any generic thermal printer to be used), but their larger form factor makes them a definite UX downgrade.
Northern started issuing them instead of cardboard tickets a few years ago and other TOCs followed along after Network Rail/Cubic finished adding barcode readers to ~all ticket barriers.
I understand why: paper barcode-only tickets are significantly cheaper than cardboard tickets (their lack of a magstripe allows any generic thermal printer to be used), but their larger form factor makes them a definite UX downgrade.