Programming languages have killer apps that define how people engage with them. Perl was "instead of using shell scripts with sed/awk you can use a 'real' programming language." Java was "here's C without so many footguns." C# was "here's Java without having to deal with Sun/Oracle." Dart was supposed to have a killer app around web development and didn't make it. Even something like Flutter has incremental improvements over stuff like React Native, Xamarin Forms and other cross-platform UI toolkits. There's still no killer app there.