Why does Steem fail to peg Steam Dollar to the U.S dollar? According to their whitepaper,


  Steem Dollars were designed as an attempt to bring
  stability to the world of cryptocurrency and to the individuals who use the Steem network.


Unlike its purpose, however, I can see that it has severely fluctuated in recent months especially between last Dec and this Jan like most other alternative coins from CoinMarketCap.

I guess it's cause it has been able to be traded in some exchanges like UPbit for some reason outside of Steem ecosystem so it has lost control of Steam Dollar even with the help or responsibility of their witness (but I've never traded crypto currencies before so am not sure whether it's possible to trade Steam Dollar on that exchange).

Do you have any other opinion?
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.