The text never fully reveals why it is that Amanda married her husband. The most information that the audience is given comes in Scene 6, when Jim finally pays his visit to Laura, and Amanda reveals to him something about her background and the choice that she made of her husband, revealing the following details to Jim and to the audience:
But man proposes--and woman accepts the proposal! To vary that old, old saying a little bit--I married no planter! I married a man who worked for the telephone company! That gallantly smiling gentleman over there!
Although no specific detail is given, it perhaps can be inferred that Amanda married her husband because she was swept off her feet and fell deeply in love with him. The first phrase, "man proposes--and woman accepts the proposal," suggests that there was something automatic, unthinking and unquestioning about Amanda's acceptance of her husband, and this normally only happens when somebody is deeply in love, so in love that they don't really use reason to aid them in their decision making. Whatever the reason, the importance of the father as the "fifth character" Tom refers to that lurks in the background of the play cannot be understated.