I don't know why she's not going to attend. There are no facts to the issue. There's probabilities, there's speculation.
My speculation - for what it's worth, and it's not worth any more or less than anyone else's is this.
Many here dislike the woman, some intensely - some, like Jeremy Clarkson, have made it quite clear that they hate her.
She knows this, and so does Harry.
The media are obsessed with reporting every detail, no matter how irrelevant, of her life when they find such to report.
She also knows this as does Harry.
If she comes, the media will have a frenzy of speculation, column inches by all the usual contributors - she will be watched and every move, smile, non-smile, gesture, will be interpreted in some way by 'experts'. Among all the other photo's from the event, there will be lots focusing on her - and then the tabloid readership will accuse her of hogging the limelight.
If she doesn't come she will - as she was in the Independent recently - be accused of snubbing the King.
So either way, she's not going to endear herself regardless of what she chooses to do. She's already damned.
What would you do? Under the circumstances, knowing in advance that I was already 'damned'... I'd stay home and celebrate the child's birthday. She's got nothing to lose.