By and large, Frost has employed blank verse in this poem, which allowed him to make use of iambic pentameter using first stressed syllable followed by stressed syllable. For instance, let us take look at the following line:
And then—the watcher at his pulse took fright. (30)
However, the poet does not follow this pattern throughout his poem such as in this line:
So. But the hand was gone already. (27)
This line has certainly not used iambic pentameter, in fact Frost has interrupted his own metrical pattern to let his readers know that they are going to see something terrible.
This to and fro between its regular iambic metrical pattern, there seem like an appropriate choice for using such meter and form.