Language can be bottleneck if there's something huge missing from it that you need, like how many of them didn't have first class support for cooperative multitasking, or maybe you need it to be compiled, or not compiled, or GC vs no GC. Go started out with solid greenthreading, while afaik no major lang/runtime had something comparable at the time (Java now does supposedly).

The thing people tend to overvalue is the little syntax differences, like how Scala wanted to be a nicer Java, or even ObjC vs Swift before the latter got async/await.