It's because of what characters people want to play.
Back in the 80s that was decently realistic burly men like Schwarzenegger.
Now it's basically anime heroes and lithe females.
The connection between strength and fighting prowess has been truly broken.
Just consider how the portrayal of Legolas and Gimli differs from book to film.
Once they were equals; Gimli's sheer brute force more than a match for Legolas speed and agility. Now Gimli is the joke while Legolas has become essentially a superhero, with his arrow-stabbing, shield-surfing, and mumakil-killing.
You look at 5E you'll see the exact same tendency.
A minmaxer would never build a strong slow character like Gimli today, when options for magical death rays and dexterity out the wazoo gives much better results for the same costs.
I bet that come 6th edition only hapless NPC guards (and a token PC build) wears heavy slow cumbersome armor. All the rest of the heroes don't let armor crimp their style.