The card readers rarely function adequately if at all. On occasions when they do work, the screen is broken meaning the customer can't identify whether payment has been requested, the amount requested, whether payment has been made, how much you are being charged, or even whether you are paying the bill of the correct till.
The cards interfere with each other and in particular with Oyster transport card in London - people use their Oyster card frequently and therefore have that in primary location: still have to pull out whichever other card they want to pay with.
In practice, people don't often use contactless cards and most customers I've observed attempting to use them express fear and wish to revert to traditional method.
The receipt is still the final part of the transaction, which customers are more inclined to wait for because they have no other way of confirming what just happened.