one reason could be it would take an extra 10min just to wait your turn in line. 

There are many places that don't accept cards. Considering that most people do not use cards here, and the amount they pay at Mcdonald's and KFC is relatively small, why would they need that ability?

The reason for not accepting credit or debit cards in China at KFC or McDonald could be many different ones, from slowing down the line, to extra cost from the charges by the bank.

Back in the States, in my business, I had to place a $ 200 deposit on machine, pay rent on it every month, have a dedicated business phone line to it (basic cost was $ 45 per month plus 13 cents per call made), and the bank also took from 1 to 2 % of each transaction.  IT WAS VERY EXPENSIVE TO ACCEPT PLASTIC, but had to have for survival.  Maybe the same happens here.

Not sure about where your at in China but I think at least KFC accepted cards in BJ. I've been regularly going to Burger King since they have that big sale going on/ whopper meal for 18 kuai. Haven't been to Mcdonalds lately so I wouldn't konw about that. Last time I was at KFC I thought I saw you could use the card there if I wasn't mistaken.