Apple users might have been wondering why Jobs' Mob did not upgrade its 13inch MacBook Pro to Intel's latest processor last week. Jobs Mob updated its range to Nehalem-based cores, as well as a die shrink to 32nm, gives these Core i3, Core i5, and Core i7 mobile processors.

However the 13 inch range stayed using the Core 2 Duo and Ars Technica claims that there are several reasons really good reasons that factored into Apple's decision.

Firstly it would cost too much for Apple to do, graphics performance and battery life would suffer and then there was the small matter of the laws of physics.
When MacBook Pro models, were first launched in October of 2008. All of them were equipped with a Core 2 Duo processor, a NVIDIA GeForce 9400M controller with integrated graphics, and a second discrete GPU.

The 16-core integrated GPU in the 9400M was far better than the integrated Intel GMA950 graphics in the Intel controllers used in previous Mac models.
However in Arrandale mobile processor family, Intel moved the memory controller, PCIe bus controller, and integrated graphics onto a separate, on-package die right next to the CPU.

This means that the processors need a less-capable and power-hungry, controller chip to handle the traditional southbridge functions. This was ok with the 15” and 17" MacBook Pros, which come with an NVIDIA GeForce GT 330M mobile GPU in addition to either a Core i5 or Core i7 processor mated with an Intel HM55 controller.

While the 13" MacBook Pro could have had a Core i5, but that likely would have bumped the cost higher than Apple was willing to go.
The faster 2.4GHz and 2.66GHz Core 2 Duo that Apple is using is cheaper than both the i3 and i5. Since it is older Intel was willing to cut Apple a killer deal to clear out inventories.

Graphics performance would have taken a significant step backwards if Apple opted to rely solely on the Intel HD IGP that comes with Core i3 or i5, so Apple would have also had to include a discrete GPU like that used on the 15" and 17" models. Besides the added cost, there's simply no room on the 13" MacBook Pro logic board to include a discrete GPU.