As an engine, Chromium has overwhelming market share, so it decreases risk to build on top of it. Choosing an alternative browser engine increases risk. Embeddability has little to do with it; Chromium (as opposed to WebKit) isn't actually that embeddable. Rather, market share is king. With a high market share, the community will step up with projects like Electron and Chromium Embedded Framework that add embeddabili…
You have your cause and effect backwards. That market share was achieved in large part by ease of embedding. Google itself started the project on WebKit because it was easier to embed and manipulate than anything else, and were pretty careful to allow others to do the same one level higher up the stack (i.e. making it easy to build your own branded browser on top of Chromium).
Google achieved market share by taking the embeddable WebKit, wrapping it in a non-embeddable wrapper called Chromium, and eventually forking WebKit to Blink in large part to make it non-embeddable and more tightly coupled to Chromium.