It could also be that they want to include a particular sequence in the first trailer so that the trailer will really grab an audience but it's taking a long time to get that sequence's special effects just the way they want them. Sometimes it takes months and months to get certain things just right. And perhaps they don't want to release a trailer without that sequence.
As for releasing a poster - even a teaser poster - I don't know what the holdup could be there. But wanting to wait until DS can get more pre-release attention makes as much sense as anything else.
142 days 11 hours 44 minutes 47 seconds until the day the Depp/Burton Dark Shadows is released(ET)!!