Good question. I've been curious about this as well. From what I understand, the interest rate is set by consensus of the active miners, or witnesses, and is not related to pegging SBD to a roughly $1 equivalent of STEEM.

The pegging is set by the blockchain and average price of STEEM providing there is enough STEEM to honor SBD redemption. The peg would not function if the price of STEEM were to completely collapse since SBD is essentially backed by STEEM and not the US dollar itself.

It seems the initial incentive of SBD and interest was to bring people into the STEEM ecosystem while not exposing every participant to as much daily price risk and the initially high inflation of a new currency. Now that the market cap of STEEM has risen dramatically compared to the market cap of SBD the debt to equity, so to speak, of SBD to STEEM is very low, implying a lower interest rate.

If anyone has a good understanding of this please share as I would like to understand it better myself.