Answer:
The first pharaoh to be buried in the Valley was Tuthmosis. Before that, pharaohs were buried in pyramids, with all the belongings that tomb robbers could dream of. A pyramid was the least safe place for valuables, and during the First Intermediate Period, almost all of them were robbed. During the Second Intermediate Period, before Ahmose kicked out the Hyksos, pyramids were robbed regularly.
Tuthmosis was smart enough to learn from the past. He wanted to protect his tomb and his next life, so he decided to hide it and keep the location a secret.