It lowers the rewards because author rewards are not shown in either SBD or Steem. You get half of your rewards in SBD, so that's simple. The other half is the amount of steem worth the dollar amount.
So, let's make it simple and say author rewards are $10 after curation. Let's say SBD and Steem are each worth $5 USD.
- You get $5 SBD, which is worth 5 X 5 = $25 USD.
- You get 2 SP (10/5) which is worth 2 x 5 - $10.
The curators are only getting paid in Steem. If SBD were closer to $1, there would be no difference in value (i.e. USD value). But it would lower author rewards, so ouch.
Also, merchants would prefer to have a stable currency to set prices. It seems like some good coder could write an algorithm that would change the SBD price on merchant sites to reflect value though.
I understand Authors making more...but I don’t see curators making less than they otherwise would if the SBD was closer to the peg. The pie has grown, it’s just that it all goes to authors from what I can see in the math.
It's all relative. The Curators are being left behind and are getting weaker and fewer. This hurts the platform.
I did some experimentation and analysis on this in my post High STEEM DOLLAR Dampens Curation Incentive but I'll give you the spoiler - I'm not bothering with serious Curation any more and lots of people aren't.