Why do people do this? Usopp situation is not comparable to Sanjis or even Robins, Sanji and Robin unlike Usopp, were doing what they did for the crew. Why do you think Usopp is the only one who apologized to Luffy????
Because He was outta pocket, out of line, out of place, he wasnt doing shit for the sake of the crew, he was hurting the crew. Usopp's disrespect towards luffy was genuine, the foundation of usopps actions is because of his earlier fuck ups in the crew and personal feelings for the ship, The foundation of Sanji and Robins actions were to save their crewmates.