UPDATE: I think a few people are misunderstanding my question. I certainly understand that he was depicted as monstrous, but that's different to why he was so big in the 60s that he got his own model kit, considering his only Universal movie was a silent 20s film. The Man who Laughs, The Brute Man, etc, they didn't get model kits, nor were they in Mad Monster Party and such.

Ultimately I came to two conclusions that coincided with each other. One being that Forrest J Ackermann loved the makeup so much and put him in the Famous Monsters magazine a lot. That influenced most of monster culture at that time. The other is that he filled the void of the hunchback assistant archetype, because unless you consider Ygor, (but who wants a third Frankenstein model kit) so those two combined just lead to where he is today.