The argument would be no different from all the products out there who design their own UI (MUX, GitHub). Our framework Frappé (and product ERPNext) has a great emphasis on minimal style, and we really wished the charts to be free of visual clutter (not have too many measure markings for instance). Something that even the customizing API should take care off. Chartjs is awesome and very comprehensive, it just didn't go too well with our look and feel :)