The optimist in me (which spends most of its time tied up and gagged) thinks the reason thees previews and the new GSL and other things WotC was said would happen are delayed is because they are putting the priority on the release of the books. Those are taking more effort than they expected, so they are letting the other things slide in order to keep the book release dates on schedule.
The pessimist in me of course believe this is all just an indication that WotC plans their project timelines like too many other organizations - through the art of wishful thinking.