Other mistake: Subtitle (English) appears to be incorrect: "He is setting the scions of criminal investigations back 1000 years" should be "He is setting the science of criminal investigations back 1000 years." "Scions" is a description of the branches of a tree. (01:11:00)
Other mistake: When Dreyfus crosses his arms, that position he does is impossible for him to have stabbed himself. He'd have had to raise his right arm up with the letter opener pointing directly towards the left side of his chest then thrust downward.
Other mistake: When Clouseau's in the nudist camp garden he's mostly dry. Somehow so is the guitar he was carrying when he fell into the pond.
Answer: In this particular film, he's referred to solely as "Commissioner" and is then Chief Inspector in every subsequent Pink Panther film in which he appears (up until "Son of the Pink Panther" [1993], Herbert Lom's last appearance in the role, when he is again Commissioner). If you're after a reason, there likely isn't an official one, though perhaps Dreyfus's murder spree and mental breakdown in this film led to his demotion.