It's not that it "can't", it's that Apple doesn't want a loud (and hot running) laptop. They also clearly see from sales history that most buyers are fine with 28W CPUs for the 13" rMBP. So, why create a (likely) compromised experience for 95% of the audience, just to please 5% that needs every ounce of performance potential in that size machine.
To give a very recent example, the Razer Blade 14" was just updated yesterday, has a 45W Skylake Quad-Core and 6GB nVidia 970M graphics, and the package is very similar in size to the rMBP 13. Yeah, it's probably hot as the surface of the sun if you're actually gaming on it on your lap or similar, but the configuration exists...just not in Apple's lineup.
It really boils down to Apple not wanting to canabalize their more expensive 15" line, and they likely felt the drawbacks to shove a higher TDP part in weren't worth it for most buyers. I'd prefer the option to exist, but also understand it won't happen until Intel releases a part that matches the necessary specs, or Apple comes to terms with CPU throttling/downclocking to make a higher-end part work.