Historically, merchants did not like to use a POS system owned by a processor because it effectively locked them into this payment processor. Processors know that its a big cost and hassle to switch POS systems - so they can charge above market processing rates to this captive merchant. And, we all know non-competitive rates can cost a merchant hundreds of dollars each month (which is why processors have suddenly embraced POS)

I believe that over time merchants will again realize this trap and seek to use systems that are processor agnostic. The good news is that there are many modern POS systems that offer lots of value added services w/o locking the merchant in with one processor.