A lot of the decisions are driven by internal Yahoo! needs. For instance, the new Yahoo! Mail really needed Rich Text Editor, but they could build their own chrome around it, so we have a really basic RTE that works, if you can build the chrome for it.
Data Table is one thing that many people have been asking for on the forums and in IRC for a long time, but it's also really complex. My suspicion is that it was waiting on DataSource to reach a certain level of maturity before pushing forward to release.
The focus on YUI3 from the team has been on the Core, and it's a really good core. Really, this is the first release that they've really been able to focus on Widgets, as most of the core widgets prior to 3.3.0 seemed to be more proof of concept.