Why did God cap the human life span?

The average human life span was in the mid 900s at one point, and all of a sudden it was limited to low 100's, with few ever reaching over 120. I was wondering what the actual reason for this was. Was it because God saw that we would become overpopulated too quickly, or did we do something to make him angry?
Update: Thank you Bill A.
Update 2: Jon M and Jeremy B also make good points.
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