Well first, Klingon honor is very relative to the klingon in question, and story being told. It seems very fickle, and what is and isn't honorable sometimes is very fluid and changing.
Also, are people in Western Civilization the same people they were 70 to 100 years ago? You can cite the UK and French Empires after WWII as examples, and even the US in its apparent downward spiral as a world leader. As a nation's world status changes, so does its attitudes and social mores. So why can't the Klingons have changed between TOS and TNG?
And actually, we did get some explanation for the Klingons changing (albeit somewhat a retcon) in TUC, when the moon Praxis blew up, that changed the entire Klingon economy. They no longer could afford their military budget, which means they shifted from an expanding empire to a world struggling to survive. And later on, the federation came to the Klingons' aid in both the battles of Khitomer and Narendra III. So, with the Federation helping the Klingons both economically and in battle, as well as opening up trade and cultural influence, perhaps the TNG era Klingons had some of the Federations' attitudes rub off on them, and they became more enlightened than the TOS era Klingons, at least relatively speaking?
Actually I think this is definitely a very reasonable explanation.
But it still feels, for me, like the Klingons being so
obsessed with tradition feels like a bit of an ass-pull. Unless (as I've theorized in other threads recently) we take it that they are all playing up the 'honorable Klingon' bit for the benefit of outsiders? Offering up a kind of Disneyland Klingon Empire, something that looks kinda like the real-deal but is really just pomp and pretense?