JS-Frameworks tend to add complexity. They extend the language by introducing new concepts (jsx, DI, additional template languages, filters, directives, collections, ...) or large inheritance trees. This often leads to confusion and steep learning curves. Frameworks like Angular, Ember or Backbone also tend to lock you into their eco systems.
It's the first framework that speeds up your development from the first seconds of use since it has a pretty small API surface and great documentation. Despite this is already pretty awesome there are two additional arguments to use MithrilJS: it's super tiny (<5kb minified, gzip) and super fast.