They are afraid that they might damage the rover's solar panels when it leaves the lander should they bump the non-deflated portion of the airbag. I believe that even though the risk is small, they are treating it as a big risk to be on the safe side... Ie. They are doing what they can to exit the rover exactly as it was originally designed, despite the fact that they designed the rover in a way that would allow it to exit the lander in many ways other than this optimum method...

My bet is that they are taking their time with all of this as a huge part of the mission is to study the lander itself so that knowledge can be applied to future lander missions...In the first few days of the mission, developing an understanding of the engineering characteristics of lander and rover themselves is probably more important to the team than the "scientific" returns, which will come in mass after the rover has been thouroughly put through its initial tests.

Scott Evil