Why could the poet not travel both the roads?

The poet could not travel both the roads, in the poem, "The Road Not Taken", by Robert Frost, for two reasons. First, being only one person, we can only take one path. Even if we could come back another day, neither ourselves nor the path is the same path as it was before. The act of having made a decision and traveling along one path has changed who we are. Therefore, even if the first path remained in stasis, we could not go back and travel it, as we are not the same person who made the choice to begin with. Secondly, once a decision has been made, we travel forward, and we cannot travel back again. Life's choices are like the fork in the the road. We examine the options for each choice, and we use our findings to make a choice and select a path. Having chosen a path, we cannot make the choice a second time. In the fourth and final stanza of the poem, Frost states,
I shall be telling this with a sigh
Somewhere ages and ages hence:
Two roads diverged in a wood, and I—
I took the one less traveled by,
And that has made all the difference.

In this manner, Frost acknowledges that the path he chose all those years ago has brought him to this point where he is now, and as much as he'd like to, there is no going back to make the choice again.

Don’t we all wish we could be two places at once? The narrator of Frost’s “The Road Mot Taken” deeply wishes to explore both roads ahead of him in the woods but states that, “knowing I could not travel both and be one traveler,” he instead was forced to make a choice.
He did the most anyone can do; he studied both roads, tried to imagine what might be down both, and pondered the fact this one choice will lead to others, and that he will likely never have the chance to go back to the beginning, where the roads diverged, and explore the second road. We can tell that he is already saying goodbye to one path, mourning the loss of the possibilities that might have lain down it.

The speaker addresses the reason that he cannot take both roads in the third stanza of the poem. He says, in part,

Oh, I kept the first for another day!Yet knowing how way leads on to way,I doubted if I should ever come back.

In his mind, the speaker is "sorry [he] could not travel both" roads that he sees in the woods, and so he likes to think for a moment that he could come back and try the first of the two roads at another time, some day in the future. However, he knows "how way leads on to way;" in other words, the speaker realizes that one choice leads to another, which leads to another, and so on, so that one doesn't get a chance to come back to the original position and make the choice again. The speaker doubts that he'll ever be back again at this particular crossroads because his choice here will necessitate future choices so that he will be led far away from this one.