Lots of theories have been thrown around as to why the 15" and 17" MacBook Pros got i5 and i7 processors while the 13" models stayed Core 2 Duo. Predictably, these theories range from somewhat plausible to tinfoil hat territory. Someone asked Steve Jobs why the 13" MacBook Pro still used the older processors, and the recently chatty Steve replied, "We chose killer graphics plus 10 hour battery life over a very small CPU speed increase. Users will see far more performance boost from the speedy graphics."
That's what you'd expect Jobs to say in defense of his company's product, but is there any truth to it? Was it really necessary to eschew the Core i5 processor in favor of improved graphics? As it turns out, yes. Ars Technica did a detailed examination of why the 13" MacBook Pro didn't get a Core i5 upgrade, and they discovered that in order to cram an i5 processor onto the 13" MacBook Pro's relatively tiny logic board, Apple would have had to utilize the integrated graphics processor Intel bundles with the i5 processor. This Intel graphics solution would have offered worse performance than the last-gen MacBook Pro's NVIDIA GeForce 9400M integrated graphics, so the only way Apple could have avoided degrading graphical performance was by mounting a discrete GPU on the logic board -- and there's simply no room on the 13" MacBook Pro's logic board to fit yet another large processing unit like a discrete GPU. Enlarging the logic board to make room for a GPU would have meant filling space used by the MacBook's large integrated battery; when you factor in the additional room needed to cool the GPU, you need even more room taken away from the battery. Then, in order to power that discrete GPU, battery life would suffer even more.
In the end, engineering constraints led Apple to stay with Core 2 Duo processors in the 13" MacBook Pro for now. Cramming an i5 CPU into the chassis would have been possible, but it would have required sacrificing either graphics performance or taking a huge hit to battery life. Instead, graphics performance for the 13" MacBook Pro has improved by as much as 1.8x the performance of the 2009 model, and the 10-hour battery life is on par with the iPad, which is pretty stellar performance for a traditional notebook computer.
Even though Core 2 Duo is an older architecture, the 13" MacBook Pro is hardly hobbled by it. Both processor options, 2.4 GHz and 2.66 GHz, are more powerful than the two-year-old 2.6 GHz Intel Core 2 Duo T9500 CPU sitting in my Early 2008 17" MacBook Pro, and my machine is far from slow at even the most processor-intensive of tasks. It's likely the next MacBook Pro refresh will see these engineering issues ironed out and i5 processors making their way into the 13" MacBook Pro, but the current model remains a perfectly capable machine even without Intel's latest processor architecture.