Kate Middleton asked her friends to call her Catherine sometime after she dated Prince William. Her real name is actually Catherine Elizabeth but the public has been used to calling her Kate, her informal name, because of the press.
The Duchess of Cambridge, however, apparently wants to dispense with Kate because the British tabloid press dubbed her as "Waity Katy" when she was still dating Prince William. The couple was in a relationship for a long time that royal observers wondered if Prince William had the intentions of marrying his girlfriend. He had to make Kate wait for a proposal for years, hence the moniker.
According to reports, Kate hated "Waity Katy" because of what it alluded and it upset her mom, Carole Middleton, as well. So, she told her friends to use Catherine, which is how Prince William actually calls her.
An expert, however, said in a report on Vanity Fair that Kate could very well have an advantage if people keep addressing her as "Kate Middleton," especially from royal followers in America. Boston University professor Arianne Chernock said that the name Kate makes the Duchess of Cambridge more relatable to commoners. It represents her middle-class origins, which is one of the reasons why the public loves her.
Other royal experts said that Kate is still referred to by her informal name because not a lot of people are aware of the many royal titles they carry. The Duchess of Cambridge is also formally Princess William of Wales, if people were more aware of the royal rules. However, this is a complicated matter for some of her fans and friends that calling her Kate would simply do.
A member of the British press also revealed that the palace's PR team encouraged them to call Kate by her formal name of Catherine after her wedding to Prince William. However, that request was pretty much ignored and most of social media also prefer calling her as Kate Middleton. Some actually use Duchess Kate to make it sound more regal but she is officially known as Catherine, the Duchess of Cambridge.
Royal experts, however, said that Prince William and Kate aren't protesting much about the confusing names and titles. It does help that Kate's identity is connecting to the masses, much like when Princess Diana was favored by the public. It makes the British royal family stay relevant so they can reach more people when they're raising awareness for their causes and advocacies.
