On iOS, Chrome has to use Apple’s WebKit rendering engine instead of Google’s own Blink engine. Because of this, Google didn’t add the Chrome for iOS code to the Chromium code base before. Supporting both rendering engines in the same code base, after all, adds quite a bit of complexity.
Over the last few years, though, Google’s team worked on making the necessary code changes to add the Chrome for iOS code to Chromium. That work is now complete and developers who want to compile the iOS version of Chromium can now take the iOS code from the Chromium repository and do so (assuming, of course, that they run OS X and use Xcode).