I wonder if there was some kind of economic inflection point — it felt like that in my memory, too, where it felt like NiCad was advertised more as new thing in the 80s before getting replaced with NiMH. I wonder how much that perception was steered by what was common in the car battery space since that was probably the most known rechargeable battery for a long time.
My initial thought when I saw your post was "weren't these just a relatively contemporaneous improvement on NiCad batteries?"
Checked Wikipedia and nope:
NiCad - Invented in 1899 and commercialized in 1910.
NiMH - Invented in 1967 and commercialized in 1989.
I had no idea there was such a long gap between the two.