History and tradition. This goes back to the 50s when the automatic first became available as an extra cost option on mainstream American production cars.

Back then a clutch disk was worn out at 25-30,000 miles. As the suburbs expanded the commute got longer and the need to replace the clutch got more frequent. This was in an era when you knew that a Ford water pump would fail at 25,000 miles, a Chrysler starter would fail at 20,000 miles and a GM timing gear would come apart at 23,000 miles. Generators and brake linings had similar life spans. The time between rebuilds for automatics started to be longer than the life of the car (about 7 years in the late 50s) so the automatic looked like a better financial deal.

Wives and teenagers started driving. There were few driving schools so the family patriarch had to teach them how. This uncomfortable situation led more men to buy automatics so they didn't have to teach the subtle feel necessary to get a manual transmission car started.

Before WW2 most men grew up on farms or in trades and were familiar with machinery and it's operation. After WW2 the percentage of men who never got their hands dirty grew rapidly and their unfamiliarity with machinery made automatics a natural choice.

As the number of car buyers who had never learned to drive manuals dropped the cost to car makers of offering the manual option rose to the point that they dropped it.