I suspect improvements in communications media. Every day, the average man sees many other men who out-compete him, with no hope of that ever changing. He sees men who are richer, taller, stronger, smarter, etc., in ever increasing realism and "engagement". Beating them is impossible, so the correct strategy in the environment of evolutionary adaptedness would be social submission and hoping they are generous. It seems plausible that there's some biological mechanism that lowers testosterone to aid this process.