Update 2: core ipywidgets now work in Colab, as do many custom widgets! In particular, the base, controls, FileUpload, Image, and output widgets all work in colab. See https://github.com/googlecolab/colabtools/issues/498 for more details.
(Tweaked original answer): ipywidgets don’t Just Work with Colab: we have a different security model, wherein each output is in its own iframe (with a different origin than the main Colab page). This prevents ipywidgets from working without changes on the Colab side.