Good question. Firstly, your is an open source implementation by a non profit organization, as chromium is built by Google, the ultimate control of the browser is with Google. read more
And also, chromium utilizes many proprietary works or extensions to run the browser, as a result tor opted for Mozilla Firefox, run by Mozilla which is a non-profit company globally trusted by many people. read more
It is not as if the Tor developers don't know what Chromium is, this issue has risen before, and as always, there are more variables to consider. You would have to compile an extensive list of reasons, with proof, for them to even consider doing this, right now, it seems as if you just like Chrome based browsers better than Firefox, and you want to be accomodated. read more