Well Charles was The Prince of Wales and his marriage would be a State occasion. William's isn't so I guess that is the difference.
Still I would imagine the President and First Lady in year's past would have had an invite. But giving the tackiness of the current First Couple I can understand why they were not invited.
Tackiness? Understatement.
Michelle put her arm around Queen Elizabeth and patted her on the back. Very much against protocol.
The Obamas are vulgarians.