Aren't mutations random and thus usually result in weaker/worse versions.
Like 95+% of mutations result in the death of a strain, 4-5% result in stagnancy, 0.5% result in it being more effective or something
Making those numbers up but it's like mutation is a normal, mundane aspect of virology which isn't some sci fi horror fare
Basically, yeah. A virus needs its host to survive, so strains that are super lethal can't compete effectively with its less lethal mutations.