The primary motivation for Flexx is the undeniable fact that the web (i.e. browser technology) has become an increasingly popular method for delivering applications to users, also for (interactive) scientific content.
The purpose of Flexx is to provide a single application framework to create desktop applications and web apps. By making use of browser technology, the library itself can be relatively small and pure Python, making it widely and easily available.
Libraries written for Python, but not in Python have a much harder time to survive, because users don’t easily become contributors. This is one of the reasons of the success of e.g. scikit-image, while e.g. Mayavi has a much harder time attracting developers. Since Flexx is written in a combination of Python and PScript, its user community is more likely to take an active role in its development.