Ethan Frome is an odd novel, as it eschews many of the things that novels thrive on. Namely, there is no progression. Novels usually establish two things early on in the story: a goal and the path to it. The story follows the main character as he or she follows the path and achieve or fail at their goal, learning a few things about themselves or maturing or what have you. In Ethan Frome, however, the story begins long after the goal has crumbled out of existence. It requires the narrator, and thereby the reader, to travel back in time in order to see the story properly. Even then, the story doesn't follow Ethan Frome in his journey to succeed. It only watches him fail again and again as he struggles to start his journey. But he never does. And that's the beauty of it.
Ethan Frome is a story about stagnation, stagnation and failure. Ethan spends the entire story failing to achieve his one goal: to escape from his wife, Zeena, and embrace his new love, Mattie. When he acts instead of merely considering his situation, it ends with him no better than when he began. He fails at courting Mattie before he even starts. He fails at avoiding Zeena's wrath for subverting her control. He fails at countering Zeena's demand to send Mattie away. He fails at hiding the pickle dish that the cat broke, a symbol of his disobedience. He fails at running away from Zeena, as he's too poor to even afford a train out. He fails at trying to procure money, as he refuses to borrow money on false pretenses. Finally, he fails at committing suicide with Mattie, which prevents him from succeeding in anything for the rest of his life.
As for why Ethan must fail, one may say that it's because he is fighting against society. The struggle between the individual and society is a common theme of the late 1800s and the early 1900s. The upper/middle class, where most of the writers came from, were starting to rebel against the sensibilities and such that characterized the upper/middle class of the mid-late 1800s, with mixed success. Thus, you get more books that question the value of things like marriage and customs. And Ethan Frome certainly falls in that category.
Everything that would normally be considered good is turned against Ethan. His familial obligation to his chronically sick mother requires him to stay on his family farm, limiting his options for the future. His marriage, which is shown to be a gut response to the death of his mother, is cold and loveless, becoming another chain that keeps him bound to his farm. He can't even leave his wife because divorce still isn't socially acceptable, nor is leaving a sickly woman (even a cruel one) to fend for herself. Thus, society and marriagehood and the household are shown to be cruel shackles rather than the tenents of good life.