Yes, it is classy of Carter to jump in early and announce his intention to attend. Still, you and others should re-read the article. The inauguration isn't for another 4 weeks, and both Clinton and Dubya have said they would announce their decision after the first.
My guess is that Dubya is spending the Christmas season with his family, as he always has. Especially so with his father's health failing.
Clinton is probably still ducking flying objects in his house, waiting for Hitlery to get over her hissy-fit to end all hissy-fits.
I can see no reason for any delay, except to show their displeasure for Trump. After all, this is a presidential inauguration, not some pick-up softball game.
It is an insult that Clinton or Bush are trying to decide. It is customary for former Presidents to attend and very childish to have to “decide”. It should be a given!
The Inauguration is Jan. 20 -- long past the Christmas Season.