Because they just lost the moon Praxis and the Klingon Empire is crumbling
The question seems to really hit the core of the movie's plot. Right away, we know the Klingons are in real trouble:
SPOCK: Jim, there is an historic opportunity here.
KIRK: Don't believe them! Don't trust them!
SPOCK: They're dying.
KIRK: Let them die! ...Has it occurred to you that this crew is due to stand down in three months? We've done our bit for King and Country. ...You should have trusted me.
But as Kirk shows, not everyone is ready to simply give up the war. Remembering that the Klingons Kirk hosts are basically diplomats trying to save their empire, yet the dialogue remains ... unpolitical:
CHANG: Come now, Captain, there's no need to mince words. In space, all warriors are cold warriors.
AZETBUR: Inalien... If only you could hear yourselves? 'Human rights.' Why the very name is racist. The Federation is no more than a 'homo sapiens' only club.
CHANG: Present company excepted, of course.
KERLA: In any case, we know where this is leading. The annihilation of our culture.
So we are seeing Klingons in a new light because they are in an unprecedented situation and are trying to solve things diplomatically with their former enemies. As for Klingons still being Klingons, lets us not forgot (massive spoiler warning):
SPOCK: The Klingons have a new weapon. A Bird-of-Prey that can fire while cloaked. She torpedoed Gorkon's ship.
So its not like all Klingons think peace is the way out.
Out of universe, remember that the movie's plot is meant to parallel the end of the Cold War. Hence the angle of diplomacy instead of some kind of all out war (which might be more "Klingon" like). Also, this is not exactly the first time Klingons have changed in Star Trek: