Until recently[0] Dart had pervasive nullability, that is, all types `T` are implicity `Option(T)`. Which in my view was completely unacceptable even in 2011 when it was initially released.
No idea, and I wasn't judging, it's just that on average you'll see 12 languages on articles / news / threads before you see mention of Dart. I heard flutter was solid and quite used but "socially" the mentions are rare.
Because depending on what you want to do, Typescript and Kotlin are better languages with superior tooling, adoption, and community support.