They were not alway heavier, the Early rims that I
am aware of were by Martano, usually light rims with
no eyelets. Other rims which were common were
Fiamme yellow and red label rims, VERY light tubular rims,
fiamme Ergals, a more all rounder type rim.
Nisi Sludi and Countach both pretty light.

Most of the heavy steel rims were from cheaper bike boom
bikes, the better bikes had good rims.
There were clincher rims available during 70's and 80's
Champion Hi-E was a good example.

Marty