I thought it was due to the people designing the package managers. The people who run the repos Stallman-like end up shipping IceWeasel and Chromium and telling long boring stories to someone who just wanted Firefox and Chrome.
Chrome is not open source. Chrome tracks you. Chromium is open source and does not track you. Other than the slight branding differences, a user would not know the difference.
How are these slight changes an issue?
I'm pretty sure neither Chrome nor Chromium 'track you'. When you sign in to your Google account, then they keep track of your searches, to provide things like auto-complete, remember your book marks, and provide hints, location services, etc...
There's a huge difference between tracking you personally, and reading some information, keeping it in a database and providing you a better service later... (not going to lie, yesterday my boss sent me an email for a meeting, and GMail then automatically made a reminder for me and pushed it to my phone via Google Now - pretty damn awesome if you ask me).
The only differences I've seen between Chromium and Chrome is that Chromium lacks the Pepper plugin by default (which provides Flash), and the Hangouts binary blob. All the 'tracking' if you really want to call it that (I personally wouldn't), happens through Google services, not Chrome per se...
If Debian were subjected to the wishes of the authors of every piece of software they distribute, there would be no Debian.
Important things like good and efficient dependency management and library reuse are far easier to achieve when all the relevant sources are available to packagers.
IceWeasel and Chromium visually are Firefox and Chrome with different icons and you'll find them if you search their "real" names in the package manager. Not so hard, right? You can find about why they exist if you are interested, it's not required.
Except for the color of the icon and the lack of support by default for AAC, H.264 and MP3 (I have support for those 3 in Chromium in Arch), do you find anything relevant that will make some non-expert notice they are using Chromium and not Chrome?
I hadn't even noticed Chromium doesn't have support by default for those codecs as I have used HTML5 audio and video without an issue. Really, for a non-expert Chromium is just Chrome with a blue icon.