We didn't go into it in the news post, but one of the big benefits of this new format is that we can sync GeglBuffers (how pixel data is stored) automatically between GIMP and the new file. So you'll get fast auto-saves as you edit, which we couldn't do with OpenRaster. Also, the OpenRaster standard doesn't support all the features of GIMP/Krita (like non-destructive filters for instance).

(Also, currently our OpenRaster support is a plug-in that calls our PNG plug-in to do the rendering, so we'd have to rewrite it to be a core process if we wanted to use it as our main project file).