There was a lot of glitches the stacked team had to work through. If beta 1 went straight to ES 1.2 we'd see a ton of people outraged.
I myself observed some significant ones that the stacked team resolved.
Mark and the stacked team do not want to give a commitment because they don't actually know when it will be fully ready. I don't blame them one bit. Unexpected issues kept popping up. Not only would releasing a stable when it's not ready annoy the customers, but would create a huge mess for the stacked team for possibly any site that the new version gets installed on. Does anyone remember the Joomla 3.2.0 fiasco? Users were locked out of their site. While it wouldn't be quite like that, in some ways it would be (installations breaking, blank pages, ect).
The stacked team of all people don't want to let the customer down which is why they are trying to be as honest with their answers as possible. Want to know how long it takes for future releases? Observe the work flow and it will give you an idea. The number of features also effects it. ES 1.2 is a huge release in the series.