Graphite is built to be a programmatic data processing pipeline that takes the form of a render engine and WYSIWYG editor. You'll be able to write custom code for every part of the system.

And because it's time for a fresh start. Sometimes you can't turn around a heavy ship, and that ship doesn't want to be turned around. It's easy to write a sentence like that, but once you actually think about it, how does an outsider with a good idea and a capability to execute on it somehow approach an existing project and decide to "take it over"? That would be neither viable nor would it yield a desirable outcome. We're building something fundamentally different from Inkscape that just so happens to eclipse it.