Historically, the code for Chrome for iOS was kept separate from the rest of the Chromium project due to the additional complexity required for the platform. After years of careful refactoring, all of this code is rejoining Chromium and being moved into the open-source repository.
Due to constraints of the iOS platform, all browsers must be built on top of the WebKit rendering engine. For Chromium, this means supporting both WebKit as well as Blink, Chrome's rendering engine for other platforms. That created some extra complexities which we wanted to avoid placing in the Chromium code base.
There is no Chrome for iOS. It doesn't exist. Just because it has a Chrome-like UI doesn't mean it's Chrome. Chrome is the whole package - UI and engine. Without the engine, it's not Chrome. I understand Google wants to leverage the brand recognition, and I know I'm splitting hairs, but until Apple allows competing browser engines, iOS only has one browser, with a bunch of skins.