Why did Prince Harry make the Diana Awards wait until 1am?

According to People,Williame departed the event around 8:30 pm. It wasn't until midnight—or roughly five p.m. Pacific time—that Harry started his informal video call with the night's award winners. Against a backdrop featuring a wood-paneled ceiling, Harry expressed he was 'seriously impressed' by their efforts and made light of the late hour. 'Enjoy the rest of your evening, if that's your plan,' he quipped in footage shared on his and Meghan's website. 'Don’t get into too much trouble if you're continuing on. And thank you very much for inspiring so many others while also safeguarding my mother’s legacy. I truly appreciate it.'"

Yusuf Ben-Tarifite described the night to The Telegraph.
William gave a speech at the Science Museum at 7 pm, where he also distributed awards.

After the ceremony, the winners had to travel back to their hotel near Waterloo Station, nearly five miles away, for a video conference with Prince Harry, which occurred between midnight and 1 am.
As for the late appearance, perhaps Harry didn’t want to overshadow Meghan’s launch of her marmalade stand by honoring his mother.