Considering the immense popularity of Princess Diana and the grand nature of her funeral, it would be easy to assume that her final resting place would be in the same tomb where so many famous Kings and Queens have been interred. However, that's not the case. According to the Daily Mail, the Royal Vault under St. George's Chapel is strictly reserved for members of the royal family. While Diana did have a noble background — her father was the Earl of Spencer, per Britannica — she only became a royal when she married Prince (now King) Charles in 1981. She retained her "Princess" title after her infamous divorce from the prince, but she was still legally a private citizen when she was tragically killed in a car crash in 1997. 

Her brother, Earl Charles Spencer, arranged for Diana to be buried on an island on the grounds of Althorp Estate. It was far enough away from the palace to protect the site from crowds and vandals, yet close enough for her two young sons to visit and mourn privately. The estate itself is open to visitors during the summer (via Parade), but the gravesite is available only to family members. People wishing to leave tributes for the princess can come to a small temple dedicated to her, on the estate's lakeside. Visitors are also welcome to view the statue dedicated to Princess Diana on the grounds of Kensington Palace.