Because it's not. But it will be soon. Three months at the least, a year at the most. Just stick with the app.

Japanese from English speakers isn't available on the website version as of currently, but it's still in the works (note that you can already begin the course on iOS and Android).

For further information regarding the course, please see this Japanese from English course's incubator page via at here:
https://incubator.duolingo.com/courses/ja/en/status

its only available for mobile....... :(

Because it's only for mobile.