The LEGO Direct team's answer to this question is on brickset:
Since its release in 2010, 10213 Shuttle Adventure
has proven to be very popular! Although the model was designed for a
16+ target audience, our consumer call center has brought to our
intention that many younger children have been building and playing
with the model. As a result, the Direct team has decided to look at
the model once more to see what changes could be made in order to
provide a better build and play experience for a slightly younger
target audience. 10231 Shuttle Expedition may look similar to
Shuttle Adventure in many ways, however its build is very different
and offers several advantages.
- The fuel tank has been reinforced with supports to strengthen the assembly
- The landing gear is strengthened to function better across many different floor types
- The Cargo doors can now be more rigorously opened and closed
- The entire build has been reworked to limit opportunities for misplacing elements
- Astronauts now have wigs in addition to their helmets (1 male, 1 female). Face prints are also different.
- The new set now has 1,230 elements
- New Satellite can be securely positioned on the new mechanical arm 10231 Shuttle