Because I could not stop for Death
The carriage in which Death and the speaker ride is a metaphor for the way in which we make our final passage to death. If you want to be literary about it you might think of Dante's Inferno where the souls are ferried by boat into hell. Or, on a more familiar level, it's pretty common for a hearse to carry a coffin to its grave. The carriage in this poem is the mode of transportation to the afterlife. We have to get where we're going somehow.
- Lines 3-4: In line 3 we see that the carriage holds Death and the speaker. But the hint that the carriage is more significant than plain old transportation comes in the next line, where we discover the carriage also holds "Immortality," another example of personification. This should really tip us off that the carriage is a pretty special vehicle that will carry the speaker to the next world.