Because part of releasing a new, backwards approaching, IGP in the 13" MBP required saving face for both its MacBook "PRO" name and Intel's IGP capabilities itself.
If the resolution is upgraded to 1440x900, the IGP is going to perform worse in comparison to the prior 13" MBP...
I also fear Apple's ridiculous 10.6.7 downgrade was somehow to show the MBA's IGP isn't as bad as it is going to be with SB IGP. Look at OpenGL performance on it, as it dropped 30% from 10.6.6. Now, we have seen Apple screw these things up before, but they also market their new products based upon prior products and list an OS X version tested on the prior gen. If they reverse course with 10.6.8 or 10.7, in the new MBA, then they might show only a 20% loss in IGP performance vs. the prior Nvidia 320m... when in reality, it might be more like a 50% plus loss in performance.
The big thing here, that NOBODY likes to think about is the 13" MBP uses a standard voltage CPU, while the MBA will use either ULV and LV or just ULV depending on who we believe. The ULV SB IGP operates at a greater than 50% loss than the Nvidia 320m. We can see this from competing products, that yes are running Windows but still have better OpenGL capabilities in the first place.
I think the big advantage to this downgrade will be buying clearance and refurbished Nvidia-based MBAs for 25% discounts... Unless Apple somehow fits a standard voltage SB CPU in the 13" MBA, I think most will be better off with C2D and Nvidia 320m at discounts.
Apple has been down the path of using a low voltage Intel CPU and IGP in the MBA before, and it was the worst Mac created since the Intel transition. It wasn't until Nvidia 9400m that the MBA became even usable. Yes, the SB IGP is better than prior Intel IGPs, but it's still utterly disappointing in LV/ULV variants. I guess the smart buyers will be buying clearance MBAs with Nvidia 320m and skip Sandy Bridge for a more reliable Ivy Bridge model. It depends on how each person uses the MBA, but I believe the vast majority are much better off with Nvidia and C2D. I just hope Apple doesn't destroy the MBA brand to try to make Intel's inferior IGP work... especially in LV and ULV variants.