The Original Series was stared in 1966. There was no green screen to for a green suit and almost all character design was... very simplistic and low budget across all TV. Just look at TOS Klingons as an example lol

[–] Little8Lost@feddit.de 4 points 2 years ago

Yea, as the DS9 Crew did go back in time worf was asked why they looked so different and his reaction was simply: we dont talk about it ^^
I think they have a somewhat explaination in one of the newer series where soong had some stronger klingons that looked human like and that this "visual softening" carried over to others