Clinton was obligated by tradition to attend, not because she was the opposing nominee, but because she was a former first lady. And...well, she really didn't want to go.
So she did what all of us do when we want to cancel plans...she asked if her friends were going or not.
She called George W. and Laura Bush...
...and she called Jimmy and Eleanor Rosalynn Carter.
They were all attending, so Hillary hit "going" on the metaphorical Facebook invite.
Basically, post-election Hillary Clinton is actually all of us.