I feel like JS mainly has transpilers because people are locked in to the language, which is not true of python
babeljs for example used to be called `6to5` so i don't think it's lockin that causes this desire, but it's certainly true that the options for clientside programming languages that are widely deployed are fairly slim. i don't think they're related though.
People that want to write code that runs in a Web browser are locked in to Javascript, so they're "locked in" to JS; I can't think of a platform where the only option is to use Python.