For Intel's previous mobile CPU lines, they have offered 3-4 classes of chips: standard/full voltage (~35W), medium voltage (~25W), low-voltage (~17W), and ultra-low-voltage (~10W). Clockspeeds on standard and medium voltage parts are nearly comparable, with SV parts going a step or two higher than MV and MV otherwise just being highly binned chips. Meanwhile clockspeeds quickly start scaling down for LV and ULV.
Intel is currently only offering SV and ULV Core i3/5/7 parts, which means you either put up with a 35W chip, or you get a chip that only runs at 1.06GHz. However Apple has always used MV parts in the 13" line in order to preserve battery life and to keep heat down. Without a MV i3/5/7 chip from Intel, Apple doesn't have a suitable chip to put in to the 13" MacBook. Worse, Apple needs an OpenCL-capable GPU (which Intel doesn't offer), which means there also needs to be an allowance for a discrete GPU when it comes to the i3/5/7.
If you look at the reviews for the existing i3/5/7 laptops, they're almost universally 14" or bigger. For the few smaller models they all have terrible battery life and often have awkward batteries that protrude from the laptop itself due to size.
You're not going to get a core i3/5/7 CPU in to a 13.3" laptop with today's technology and meet Apple's high standards. Intel simply doesn't offer a mid-power chip suitable for such a device.