H&M announced they were both on their way to Scotland, and when Meghan didn't go it caused a whole lot of unnecessary public discussion and speculation.
We now know that Harry was arguing with his father about why Meghan wasn't invited, while his mother was dying, and was only appeased when he was told Catherine wasn't going, most people believe that Catherine took one for the team by staying behind.
So why would anyone have anything to do with them during that period, let alone talk to them in the car on a 30 second car journey. It was already very magnanimous of William to invite them to join him and Catherine on the walkabout.