[Pixel C General] cheep5k8:
Personally, and I don't have extensive data to back everything up (Ars did a more thorough research, but then again, I went deep into the code and ChromeOS gerrit, etc.), this is my opinion:

I think until early mid-year 2015 the Pixel C was still running ChromeOS in Google's labs, and it was well planned to ship it that way. Somewhen around late summer, the device was adapted to masquerade as an Android device. I call it that because it still really isn't a real Android device. The kernel source is hosted on chromiumos git, and the kernel is as much a ChromeOS kernel as it is an Android one.

But why the change? We can mostly just speculate. I didn't find any direct evidence in git or gerrit, and I doubt the developers really had much of a choice in that. I'm also sure the reason wasn't a technical one; the Pixel C would well be able to run ChromeOS as is. Maybe someone will even try to port it.

It was most likely a business decision. Maybe because the attempts to make ChromeOS operate touch only were not successful from a UX perspective. The device was already being developed on with prototype boards in 2014. At that time, though, it was mostly the bringup, so no real UI yet (as far as I could gather from git and gerrit). But still, you don't develop an OS/interface for a device only to conduct UX tests almost before release, only to just scrap it, so this doesn't seem to be likely.

No, I think this was a decision related to the future of Android, ChromeOS or both. Maybe they didn't want to bring ChromeOS touch to devices in order to promote Android in that position. Maybe they thought that in order to better sell the device, a less experimental, and already better known OS would be more beneficial. But this was definitely a product management decision; the developers really don't have that much of a say into what the final product, in terms of being a product, should look like.

A last question I have been pondering, somewhat as a conspiracy theory, whether this has something to do with Sundar Pichai becoming Google CEO. Not to forget, he was (or still is?) head of ChromeOS development before becoming Google CEO. It's possible, but depends on the details. He was announced new CEO on 10th auf August 2015. IMO that would have been still enough time to convert the Pixel C to run Android (the changes are not really too vast). I think it would be doable in 2 to 3 months, with a large enough team, which Google certainly has. Maybe the fallback to Android had already been planned for longer, maybe for different reasons than the final decision, and maybe some Android-relevant/compatible code was already there. That would have shortened the timeframe, in which to convert the device to Android, by a good amount, and would have made a date of mid-August for starting the move to Android realistic.

EDIT: But then, Pichai announced the Pixel C, already running Android, on September 29th. Would a conversion of the device from ChromeOS to Android be doable in just 1.5 months timeframe? Possibly, but it would definitely be rushed. Though AOSP is pretty easy to handle and run on a device if you have the right drivers; this would have meant nvidia providing on their part. Coding a small layer for the EC to accommodate Android...... Maybe this is what happened? Who knows.

What really happened, precisely, is, at this time, anyone's guess. Anyone's but Google's.