Jimmie Foxx is now the second youngest to hit 500 HR's but he only hit 34 more after that. Why??

Foxx's skills diminished significantly after 1941. Some sources attribute this to a drinking problem, while others attribute it to a sinus condition. He split the 1942 season between the Red Sox and Chicago Cubs, playing mostly a reserve role. He sat out the 1943 season and appeared only in 15 games in 1944, mostly as a pinch hitter.

Guys didnt normally play as long back in those days. Conditioning, training, etc. Injuries are a factor. The Mick only hit 34 more after 500. The 500 plateu is not as impressive today as it used to be. Guys faced better pitching, hit in bigger ballparks, pitching mound was higher. In other words, the game was more competitive. Center field in Yankee Stadium was 461. If Mickey Mantle played in today's Stadium, he would have hit at least a hundred more. The game has changed. And there's no telling how many more Willie Mays would have hit in a real ballpark facing this low rent pitching.

From having read a couple of biographies about him, it seems like his drinking problem was a large factor in his skills degrading so quickly. He also some kind of chronic sinus issues that weren't treated well at the time.

He was a great player, but by the time he hit his early 30s he had really damaged his body and abilities through hard, heavy drinking.

Injuries

2 reasons are said to be the cause of his demise: a drinking problem and even sinus issues.