Been mostly outa' touch with these cats, but going out on a limb and guessing "the reasons" may have something to do with the fact that a GPN album without Scott, Matt, Ben and Cat just wouldn't be as good.  That's not meant to be a dig at Grace, but maybe one at T Bone;-)  Glad to hear that more recording occurred with full band involvement!  I look forward to what unfolds in the spring, which is a happier and better time to be touring to support an album anyway.