There are many theories as to why it's taken the world by storm. Including, though not limited to:
*This country's rather peculiar attitude towards the Royals;
*The clunking great mistakes made by the usually robust Royal PR machine;
*How the British press didn't fall in line behind the "firm" as usual;
*How X/itter was already filling with "Where is Kate" conspiracy theories before the photograph was released, which only helped add fuel to the fire;
*The sense of community forged by joining in the circus of all this (which is why, earlier, I saw little issue with the phrase 'QAnon for wine moms' because research into how that spread from 4Chan to the real world under played just how most of it was a group of people wanting to meet up and share stories, theories, and solutions to clues, much in the way Swifties do whenever she posts cryptic references in her videos)
*The difference in coverage about Kate compared to Meghan (which is steeped in double standards, misogyny, and prejudice)
*How the media outside the UK deals with rumours about the Royals compared to media inside (and how those differences shifted)