I think it's because you still can't get Steem or SBD out of Poloniex (not sure about Bittrex) and therefore, you can't actually do anything with the Steem or SBD once you've bought it.

From Poloniex:

STEEM is currently under maintenance or experiencing wallet/network issues. Deposits and withdrawals will remain disabled until a solution is found, which may require an update from the STEEM team. Any updates must be tested and audited before enabling.