The reasons keep changing according to what you read. Family, football injury, horseback riding injury, etc. Old guys that I met told me it was at the beginning of his career and he wasn't willing to risk losing ground. But all the facts can't be known. A lot of guys were enlisting. The studios might have compelled him to stay home, etc. In the end, it doesn't matter.
As I say, these guys are professional historians, and they did a pretty good job rooting out the truth, so for them to say it's unclear says a lot. The book is great.