Because Kendall Jenner is the half-sister of Kim Kardashian (half-blood relationship). So kendall jenner took his father's surname instead of "Kardashian".

Kendall Jenner (November 3, 1995-), born in Los Angeles, is an American hostess, actress and model.

She started her reality show career by starring in keeping up with the kardashians. She appeared on the covers of Raine》《Genlux and other magazines, and took photos for Kiss, Teen Vogue and Seventeen. She won the award of sexiest actress and best model in teen choice awards in 216.