1) Gecko is harder to embed and fork. Google built chrome to be "embeddable" from the start, while Mozilla slowly made it lower priority, possibly due to the lower budget.

2) Gecko is licensed under MPL, while Chromium-Blink is available in BSD. Even if companies are going to release the source code, its easier not to be under obligation to do it properly in a timely manner.

3) A lot of websites only support Chromium-based browsers, or are at least only tested on them. While we all like an underdog success story, it makes more sense to use the better supported option.

Someone more experienced can provide better insight though.