User blog comment:B214/Volume 23 discussion blog/@comment-31299378-20170515215652/@comment-29963364-20170515221148

The same reason Rias is called the...


 * Crimson-Haired Ruin Princess


 * Princess of Destruction


 * Princess of the Gremory Clan

"Princess" doens't always refer to a "daughter of a King." It can be used to refer to a "daughter of a very rich and/or high-class family." Even daughters of big company CEO's are called "princesses."

Also, a daughter of a King is called a "Royal Princess." But people just use the less formal term "princess."