The stupid ones get caught before they have enough victims to count as "serial killers".
“The stupid ones get caught”
Frankly, it’s the geniuses who get caught because they assume everyone else is stupid. It’s the serial killers of average intellect who tend to persist because they’re not so orderly (or obsessive compulsive) that they use the same MO each time and they also tend to be predators of opportunity and impulse as opposed to predators of premeditation and planning. The deliberate criminals exhibit more patterns than the less intellectual criminals do.