Steem Dollars are designed to be loosely pegged to the US dollar. The reason is that the only thing we have to back its value is Steem. And if we kept giving more and more Steem so that people could redeem 1 SBD for the value of $1, the price of Steem would go even lower and we'd end up in a death spiral. If there was no limit to converting SBD for Steem, the price of Steem would already have dropped to under 1 cent long ago. This was one of Dan Larimer's better ideas if you ask me, although it may be confusing if you don't understand why it works this way.
Other stablecoins are either backed by a company, like Tether or USDC, or by a very complicated system with redundant collateral, like DAI. They will keep working almost perfectly until they suddenly collapse.