That's because while Steem is a liquid currency, whose value fluctuates just like any other coin, SBDs or Steem Backed Dollars were designed to be pegged to the US Dollar. So, while the price of Steem will be affected by market forces, SBDs will always be valued somewhere around $1 US. SBDs were designed this way to give businesses and other individuals who are not used to the volatility of cryptocurrencies a more stable and somewhat familiar coin to trade with.
Nonetheless, it is important to note, that the current mechanism pegging SBDs to $1 US is not perfect. That's why it has dropped to $0.8 US and increased to up $12 US in August and December 2017, respectively. This, however, is going to be addressed by the upcoming Hardfork 20 on September 25, which should make the SBDs more stable at $1 US.
Sources:
- STEEM Tokens. https://www.steem.center/index.php?title=STEEM_Tokens
- Steem Dollar. https://www.steem.center/index.php?title=Steem_Dollar_(SBD)