WOW! What an excellent question!
Mom had been living in the past for ages= Still talks about gentlemen callers, still reminiscing those old Southern days when she had what not many boys at her beckoning call- still wearing the SAME DRESS she wore for that evening, even after having two grown children.
Laura has been living Jim's fantasy from the moment she left high school. When Jim came to visit the house she STILL had the yearbook of the year when Jim was the leader of the school, CPT of the Football Team and what not. Both Amanda and Laura were utterly STUCK in the past.
Tom couldn't handle it anymore- it was a nightmare scenario and none of them, neither Amanda nor Laura had a remedy, for the past was their comfort zone. Hence, Tom simply left- may sound rude, at first, but if you really read the entire story you would more than agree with him in the end.
Tom did not pay the electric bill that Amanda told him to pay and, instead, he used the money for the Union of the Merchant Marines...when the climax situation happened in the story, which was when he brought a visitor home whom Amanda thought to be a gentleman caller for Laura, and whom ended up being Laura's fantasy crush, Jim, Tom simply could not handle the drama any longer after both Amanda and Laura found out that Jim was engaged to be married, and that the whole thing had bee Tom's way to please his mom and sister, and- well- it didn't work.
So, as he left the household, left them in the dark, and (at the very end) Laura ends up blowing the candle that was the only way to light them up, its a metaphone for Tom to say "wake up, women- get off the past and get on with the present, let go of your mental imprisonments, and move on"