Because it goes across the credit card network rather than the EFTPOS network so for many retailers (at least in New Zealand and Australia) it is a cost they would rather not have to absorb (the same reason why so few companies accept American Express). That being said, EFTPOS at least in Australia now have started to deploy 'tap 'n go' along with chip on their standard EFTPOS card which should mean more merchants open to contactless payment.
In the US, both credit and debit cards (from a retail standpoint) are many times run on the same networks (if you use a signature with a debit card as opposed to a PIN), so retailers aren't checking and trying to force customers one way or another to save on feeds. Most terminals are customer-facing, but it's a matter of training on the side of the staff operating the equipment (in the case of McDonald's, they've been customer-facing for awhile and it's more that the person ringing up the transaction didn't press whatever button to "complete" the order and use some sort of non-cash payment - once they do, the terminals light up and take whatever payment in whatever manner - if I'm sensing it's not ready, I'll say that it's going to be credit and then they press whatever needs to for enabling the terminal).
There was a long time where some places allowed you to swipe your card while the transaction was being conducted and then it would process after everything was totaled up. First Data terminals still allow this, so you can do chip or swipe before a total is entered in (NFC is not enabled in this state), encouraging "bad behavior" on the merchant side. On those terminals, if you enter the total and then press the green button, they'll accept chip, swipe, or NFC. We have these at work and found that was the cause of 99% of the "Apple Pay doesn't work" complaints.
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Plus, it makes it easy to justify not having the terminal customer-accessible, especially if PIN is almost never asked for.
It gets me when you see merchants that have the dual-PIN-pad terminals and they have both "halves" on their side of things...the Square NFC/EMV reader add-ons, too.