How many sources for change are available?
Innate / born with it - mutants, extraterrestrials.
Radioactivity.
Chemical agents - toxic waste, drugs.
Magic - Juggernaut.
Experimental alteration - aliens such as the Kree, space gods such as The Celestials, human laboratories.
Technological enhancement.
For a normal human on the street becoming a mutate with superpowers, tech, alien probing and magic are out, leaving chemical agents or radioactivity to fall back on repetitively. For the early 1960s, radioactivity best fit the constantly tense, paranoid zeitgeist, and exploitation of that allowed Lee to churn out large volumes of material without having to think too hard at all about origination.