"Because I could not stop for Death--" is an example of lyric poetry. Lyric poetry most commonly portrays the deep feelings or emotions of the poet, rather than focus on telling a story with characters. Lyric poetry goes back all the way to the Greeks, who would sing the verses of their songs as they played their lyres (stringed instrument), which is where we get the word 'lyric;' with this being said, lyric poetry has a musical quality rhythm to it.
Dickinson's "Because I could not stop for Death--" certainly has an established rhythm. The poem itself is twenty-four lines and is divided into quatrains which are stanzas of four lines each. Each stanza has an established pattern; Dickinson alternates between lines of 8 syllables on the first and third lines of the quatrain, and the second and fourth stanzas are six syllables long. By keeping the 8-6-8-6 syllable pattern on each line of the quatrain throughout the poem, Dickinson establishes her rhythm.
For example, let's focus on the first stanza. The poem begins with:
"Because I could not stop for death--" (1).
This line is eight syllables long, which breaks down into four metrical feet. A foot equals one unstressed syllable and one stressed syllable, and is a way of measuring stresses and unstressed syllables in a poem. Here is what the first line looks like broken down into metrical feet:
"Be-cause | I could | not stop | for death--" (1)
Because of its pattern of iambs (a non-stressed syllable and a stressed syllable), this line would be considered iambic tetrameter. Then, Dickinson's following line of six syllables, which makes into three iambs, would be considered iambic trimeter.
Dickinson's alternating use of iambic tetrameter and iambic trimeter give "Because I could not stop for Death--" a lovely, rhythmic quality, perhaps reflective of the rocking motion of the carriage in the poem; without a doubt, this poem is a lyric poem, because of the poet's purposeful use of rhythm and rhyme.
I count 8-5-8-5 in all stanzas except #4, that one is 5-8-8-5.