Why would it need to? It only draws 1A @ 5V. That's well within what one would expect from a high-output USB2.0 or 3.0 port.
You're also not likely to be frequently removing and reattaching the power cable to the device, so who cares if there's only one way to plug it in?
Edit: Or are you talking about Displayport-over-USB-C vs. HDMI? If you are, I, too am disappointed that the CC doesn't output over DP or miniDP. Displayport->HDMI adapters are easy to come by (and one could be packed right in to the CC box), DP is technologically superior to HDMI, and its consortium is not run by a bunch of evil thugs. ;)
Just my guess.
Your argument is sound, but you're making it four or six years too early. :)
(Edit: I should add that there is in fact a USB host controller behind that port, or at least there is per the rooting instructions I remember reading. But the product as shipped has no USB "support" for anything but 5V DC power.)
I'm guessing its a full-fledged general-purpose port that just has limited uses in the Chromecast devices (upgradeable) software.
If we cheat a little, and look at Chromecast rooting instructions [0] we get a strong indication that that port is totally a USB 2.0 port. :)
[0] http://cord-cutters.wonderhowto.com/how-to/root-your-chromec...