Why does Steem fail to peg Steam Dollar to the U.S dollar?
The price of the Steem Dollar is completely dependent on market conditions, so it's hard to know for sure. However, you are most likely correct about higher demand from an external exchange, outside of the Steemit ecosystem. Even though SBD cannot be redeemed for actual USD dollars, there is still interoperability between coins (SBD to Steem for withdrawls) that allows for a Korean Steemit user to exchange through Upbit.

Steem Dollars is also listed on Poloniex, but there has never really been a high demand for it, because the IOU-backed Tether is also listed there. If you look at the price of Bitcoin and Bitcoin Cash for November 12, you will see that there was a very quick exodus out of cryptocurrency by Korean exchanges at the time. Seeking a safe haven, traders purchased large amounts of Steem Dollars at that time, causing the price to quickly lose its dollar peg to the north.