For two reasons :
_ I don't like webjars because when I add a webjar I always have to seek the path of the .js or .css to include. When I install with bower, I don't have this issue, I just have to look in the bower_compoment directory.
_ I want to segment the most possible the frontend and the backend. Webjar is unknown to frontend dev while bower is not. Now, I m learning reactjs, in the future, it is possible my ninja web app will just be a REST service and the front end/UI stuff in a separate project (which is great I think).