Well they don't directly force you to use it, but when basically all of the most popular browsers (including the default browser of Windows) are Chromium based, that means that developers optimize their sites for that first and foremost. And combined with Google's amp protocol, which adds control for them, it means that they can dictate many terms for these other companies. It's like sure, you can use Vimeo to upload your video, but who the hell is gonna see it when there's YouTube? Same thing here, why use some other standard (such as what Firefox is doing) when the support for Chromium is that much greater, even though it's more restrictive in others.