Supply and price don't necessarily correlate in general, but they do in this case because supply dictates the haircut rule on conversions. Currently the conversion rate (w/haircut) is 1 SBD into about $0.66 worth of STEEM and sure enough the market price (looking at exchanges with a working wallet such as Bittrex) is right at $0.66.
Continually reducing the supply of SBD guarantees that the debt ratio and, therefore haircut, will decline as long as STEEM has any value at all, meaning convertibility into $1 worth of STEEM will return and so will a market price of about $1. Now it is possible that a market price of $1 will happen sooner, but that part is speculative.
BTW the above-mentioned debt ratio also controls the spigot. Other than the very small amount going into the SPS treasury, no new SBD will be produced until the debt ratio is reduced below the point where convertibility into $1 worth of STEEM is restored. The only other way this could happen would be extreme manipulation of the price feed data by witnesses. That's a theoretical possibility but very, very unlikely (both IMO and from experience).