There is a common trope in Science Fiction television that is a functional motif to illustrate and explain the difference between Earthling homo sapiens and every other species in the galaxy.
In order to make the other alien species conceptually understandable there is built into them the paradigm of monoculture and mono-species to each inhabited planet.
Michael Straczynski even made it a point in the punch line scene of an episode of Babylon 5.
In season One episode Five, The Parliament of Dreams, there is a festival on the space station where each species gets to showcase their culture and therefore their religious tradition in a ritual performance that is point on by each species to highlight and present to the other species just who they are. The Minbari and the Centauri both put on an elaborate ritual. At the end of the episode, Commander Sinclair of Earth presents to the other ambassadors who are gathered to meet this long and seemingly endless line of human races who represent the planets diverse racial and cultural mix.
The point is that all other planets in the vast galaxies are portrayed as monocultures and monoracial. It makes it easier for the show writers and producers to have the audience understand who they are seeing. That are the Narns, That is the Minbari, the Centauri, the Vulcans, the Romulans, etc. Rarely is any planet have a diverse racial and cultural mixture, except of course Earth.
I think the easiest way to understand how the Klingons are portrayed so differently in Star Trek: The Original Series, The Next Generation & Deep Space 9, and then on the new Star Trek: Discovery series is that for once we find a planet that shares something so fundamentally similar to us on planet Earth. The secret is simple, the Klingons are multi-racial.
Now, of course this is not the tactic taken by the producers of the Star Trek series.
They the writers, try to retcon all the diverse differences away with some fabricated explanations. When they simply could have done this more simply and more organically, and admitted from the Next Generations that the Klingons were a multi-racial species.
Let me imagine that the Klingons from The Original Series are Klingons from ‘the Mongolia sector of the planet’, the Klingons of Next Generation & Deep Space 9 are the from the ‘European sector of the planet’, and the Klingons from Discovery are from, say, ‘the African’ or ‘South American’ sector of the Klingon home world.
That's it. No big deal, the Klingons are a multi-racial species. That could have been the way to deal with all the different looks, but the writers just didn't think of that. They were stuck in the trap that all alien species must be mono-racial.
Gary Jaron's musings.
In my High School Art Department someone had made an ornate sign on hung it on the wall that read: 'Ignore this sign completely.' A paradox couched in sarcasm and irony. This blog is for random musings on anything and everything that comes into my head.