@andrewharland is correct, it's because voting power is driven by SP (which is linked to STEEM) and not SBD.

While we believe that in the long run SBD will actually be a more stable currency (pegged close to 1 USD), over the lifetime of our program that has been false.

Try the following: Load 90 days of price data for both STEEM and SBD into a spreadsheet. Subtract the daily Low from daily High to find the daily range, and then divide by the Open to find the daily % range. Over the last 90 days, the average daily range for SBD is 12.93%, but the average daily range for STEEM is only 12.24%.

That's actually closer than I expected them to be, but (surprise!!) STEEM is actually a more stable currency than SBD.