Not good reproductive candidates. Everyone that went in were self-destructive or suicidal in some way.

@drjekel_mrhyde said:

Why didn't the aliens just clone/copy every single person that went in?

Not everyone reached the lighthouse.

@drjekel_mrhyde said:

Why didn't the aliens just clone/copy every single person that went in?

Kinda like The Thing as in there were "levels" of Annihilation to which only in ideal circumstances were the ideal specimens replicated.

In a tertiary situation or when the creature is attacked, the transfer is violent, and seemingly viral.

@drjekel_mrhyde said:

Why didn't the aliens just clone/copy every single person that went in?

Not to mention there wasn't a goal to replicate. That was stated in the movie. The whole thing was a sped-up process of evolution within the shimmer. Since the theme centered around cancer and cancer has no objective, the alien organism had no objective either.

It was "learning", it was mixing individuals around, as if trying to understand how life on Earth works, while it was adapting and absorbing characteristics. Only late it was able to replicate, and still Kane replica became sick. And it stayed on the lighthouse, so only humans that reached there would be replicated.

In early days it was unable to replicate and probably just wiped humans who attacked it, not even understanding what they were and what they were doing, just mimicked them. Later as its area grew, it became harder to reach the lighthouse and most ppl died by the changes they were suffering or killed by monsters or killed each other.