The "not" in the title puts emphasis on the road which the speaker does not take (as opposed to the one he did take). There are many textual clues which imply that the speaker is troubled by the fact that he cannot go down the other road as well. We notice that he takes a long time to examine both roads, which suggests that he is really torn between the two options. He is aware that once he chooses a path, he cannot go back and discover what the other one offers:
And both that morning equally layIn leaves no step had trodden black.Oh, I kept the first for another day!Yet knowing how way leads on to way,I doubted if I should ever come back.
Moreover, in the last stanza, the speaker says that he will talk about his choice "with a sigh," which suggests that he grapples with a sense of regret and sadness because of the impossibility of making more than one choice at a time. Despite choosing the path which he believes is the right one, the speaker cannot let go of the idea that he will never be able to see what the other path offers.