My Unscientific Speculation: When there were only a few people walking the earth, many individuals lived much longer than average, and produced more children than average. As the earth became heavily populated with human’s, the average lifespan was sharply reduced over a few generations.
Perhaps there was less food to be distributed than before.
My speculation: Adam and Eve were created by God’s own hand and were thus genetically perfect. Lots of things can kill a person that are not related to genetics, of course, but having no genetic defects would be a big help. Genetic defects started piling up after the fall, and then God brought the hammer down and limited us to 120 years, give or take.