Observable notebooks exist. The issue for JS replacing Jupyter is that Python already has a large number of scientific libraries where the performant ones are based on wrapping C++ or Fortran code. You can handle more data and processing on the server. Also, because Jupyter gives you shell access and other languages like R can make use of it.
Language-wise, the other issue JS has is that it lacks operator overloading, which makes array handling a lot nicer. Python has magic methods for doing that. Julia, R and Matlab have that built into their languages. Julia also has it's own version of reactive notebooks similar to Observable.
And then you have a lot of scientists who already know and use Python, R or Julia. I can't really imagine a statistician who's well versed in R finding much value in JS. Javascript just isn't made for complex statistics the way R is.
JS now feels like Java back at the turn of the millennium, when people were thinking Java could just be used for everything, and it sort of was. Or any language could run on the JVM (the web has largely replaced that idea). But there's a reason for the various different programming languages. Some are just better at doing certain things. And JS is not a scientific computing language.