Its not tracking it because there's a lot of sell pressure.
Every day alot of Steem dollars are created via blogs and then transfered to poloniex or bittrex to sell imediatelly. Someone has to buy them for bitcoin, and at the moment there's not enough people who are willing to give bitcoin for steem dollars, so the price is drifting down. It can change ofcourse.
When poloniex has added steem dollars to the market price was 1.2 $ for one steem dollar (if I remember good). There was not enough SBD on the market and a lot of buyers.