According to this topic, because the Halloween event quests end on November 12th so it’s possible there’s a patch on that day that will be the patch to bring pirates to MapleSEA. Mere rumour or truth? Who knows, let’s wait until that day. ^^
*Wee little very belated update : Pirates for MapleSEA was released on 19th November instead. The news for the patch is here : Game Client Patch on 19/11.