They could've made a C API, with a thin Dart wrapper around it, allowing multiple languages to create their own bindings, like GTK did. Instead, they decided to couple a UI framework with a language that almost no one is using. Maybe because it was easier for them to implement it that way, or as an attempt to make Dart popular after developers have been snobbing it for years.

In the end, if flutter becomes the de-facto standard for mobile apps, developers will hate it for the same reason developers hate on javascript: it's not necessarily for its technical merits, although they do play a role, but because it's forced on you with no other choice.