Because the increasing split between SBD and dollar could have unexpected and undesired consequences for the entire STEEM ecosystem, Steemit INC, (or somebody who has control over the relationship with Poloniex in regard to that), decided to stop the influx of capital, so SBD pump can stop. This assumption is supported by the warning from Poloniex:

I think better explanation for this is just that Poloniex is having some problems with the wallet software and needs some input from Steem developers, but Steemit Inc is being again unresponsive (lack of focus and priorization). Or another but less probable reason is that Poloniex has found some real bugs from the wallet and Steemit devs are still trying to fix them. But it has been taking quite long time now.