It is because each SBD in circulation has a guarantee to be paid in STEEM when you cash in the SBD. Each SBD is worth $1.00 US worth of STEEM. When the STEEM price is low, then paying for SBD would be difficult. To prevent too much SBD from getting created when price is low, the blockchain is programmed to limit SBD creation or stop SBD creation. Right now, there is no SBD being created. When you choose 50/50 rewards you get STEEM instead of SBD since there is no SBD being created.