Simple explanation: They have the hardware capable of NFC payments at Sainsburys, Ingenico 3000 series. However when they had them installed the firmware didn't contain stable/release NFC reader code, or they didn't opt for that firmware to be installed. There are backend reasons why you wouldn't have that firmware as well.
They have 1374 stores, let's say 15 devices per store, that's 20610 devices not including PFS & customer services. The average firmware update takes say half an hour, field service engineers at £30 per hour. That's £309,150 in time just to enable NFC - let alone travel, subsistence etc. And what is the benefit to Saisnsburys from spending a large chunk on that? Well, very little.
That's how these people think....sod the customer haha