Why do you think Carver chose the title of this magazine for his short story "Popular Mechanics"?

Actually, Raymond Carver didn't choose the title "Popular Mechanics" himself. Originally, the story was going to be called "Mine," but his editor changed it to the title as we now know it. Popular Mechanics is a mass-market magazine which looks at various aspects of science and technology. It's especially notable for providing readers with practical "how-to" advice on such topics as home improvement. It's possible, then, that the title is being used in Carver's story in an ironic sense.
The characters in "Popular Mechanics" are a bickering couple with a baby. The man has packed his bags and is ready to leave, though we don't know why. However, we do know that this is a bitter and acrimonious relationship, full of distrust and intense mutual loathing. The woman in this dysfunctional relationship is determined to keep their baby; in fact, she's so possessive of the child that she won't even allow her partner to have a copy of the baby's picture she's just taken.
The mechanics of the title could conceivably refer to the process of the couple's intense struggle over their baby. This is a story about the actual nuts and bolts of a bitter struggle rather than its (ambiguous and potentially tragic) outcome. "Popular" could suggest the fact that this couple could be anyone. We don't know their names; we don't know how old they are or what they do for a living; we don't even know if they're married or not. The couple in the story, like their poor, unfortunate baby, are universal archetypes.