Why? May 19 falls on a Saturday next year, which breaks time-old royal protocol: traditionally, British royalty marry on weekdays, not - the horror! - weekends.
As Harper's Bazaar UK points out, Prince William and Kate Middleton married on a Friday; Princess Diana and Prince Charles chose a Wednesday. It's just what these couples do.
Choosing a Saturday isn't the only wedding tradition Prince Harry and Meghan Markle are breaking: they've chosen banana cake instead of the traditional fruit; and the press may not be present. At least that explains our lack of invite.