Boils down to two issues: most importantly cost, secondary to that, "new" image.
The main reason for redesigning the packaging of a console is if the complete product can be produced at a significant savings over the original. SCE saves about 35% off manufacturing costs in the transition between the current SKUs and the slim.
But if the case is already a cheap component, the console may well just continue to have more and more empty space in it, housed by the same shell as the chip set shrinks, rather than redesign the case into a smaller format.
But a "new" image (new physical appearance) could be key for marketing purposes if MS intends to relaunch the console next year as a new product with the debut of Natal, feasibly in an attempt to capture some of the Wii market.







