To my understanding the biggest reason GitLab chose Vue was because they were able to easily mix it in with their existing stack. That's part of what I was referring to by "drop-in". You can do the same with React but it's more heavyweight unless you plan on porting the lion's share of your app to React.

As for the others I'm not sure what they're using Vue for specifically and can't really speak to the technical reasoning behind it. I'm not saying that you can't do serious development in Vue but rather that all else equal I will guide teams to adopt React, mostly for the better developer ergonomics.