The developers doing the new content are not the ones fixing bugs. So Bioware can do both things at the same time.
Fixing bugs is rarely as easy as we like to think, seeing it only from the outside. So it always takes more time than we, or the developers, hope.
The problem is that the guys creating new content are creating new bugs and the poor "bug fixers" are never getting the chance to catch up.
Maybe it's time Bioware take the new content guys and have them help out the bug fixing guys for a little bit.