From them moment Shaddam IV ordered him to Arrakis, Duke Leto knew that it was a trap. He was even aware that there was a traitor in their ranks. Nevertheless when the Duke was attacked by
Dr. Yueh
he didn't have any bodyguards with him. Moreover, the shield generator - which one would assume their most important line of defense - was so weakly guarded that not only could a complete stanger like
Esmar Tuek
get near it but it was also possible for a single person to eliminate all it's guards.
Given this, why didn't the Atreides guard themselves better?