In addition, their reasons for deprecating their old extension API was purely technical. Even then, they were extremely conservative in their deprecation timetable, waiting long past when the old API was massively harming performance. But once they made the decision to replace it, it made sense that they would choose an existing API as a base, not make a separate incompatible API.
But do note that, while Firefox and Chrome share the same base extension API, Firefox has APIs Chrome doesn't. It's reasonable to believe that this new deprecation will just become another thing that Firefox has and Chrome doesn't.