I agree with the stoarge bin hypothesis. Before I was Elite on ATA, I was usually in an exit row in which my being allowed to store my carryon under the seat in front of me was entirely at the whim of the FA. It was also in the last boarding zone to be called. Being first in line vs. being last was often the difference between having my laptop and other in-flight necessities or being forced to watch them sent off and subjected to the whims of the Luggage Handling Gods in one my one tiny carry-on because the bins had been hogged by those who bring a dozen bags containing every possession they've ever owned since birth.