Very nicely done. Much more articulate than my answer, which would be "because he's sex on a stick." :-)
Tierney was a handsome man with a very good build by 40s standards, but it is more than that. In these early roles, he has a masculine presence that's almost breathtaking if you like that sort of thing - just pure virility, and his role in Born to Kill really plays up that appeal. I've seen comments doubting that Georgia would be swept off her feet to the point of marrying this rough man she hardly knows, but I didn't find it hard to believe at all. She was probably a sheltered young woman who hadn't encountered anything quite like Sam before.