Why are the british royal.. didn't wear their... crown?

You obviously don't live in the UK. The queen would be the only person to wear a crown, and that would usually be on her coronation day. She sometimes wears a tiara, which is a small jewelled head coronet. This she usually wears on state occasions. The Duchess of Cambridge wore one last week for her wedding, this was lent to her by the queen, who was given it by her late mother, Queen Elizabeth the queen mother.
They don't walk around with crowns on all of the time, they are quite human really.