Why couldn't the speaker stop for death in "Because I could not stop for Death—"?

The speaker of the poem has no fear of death. That's what she means when she says that she couldn't stop for it. And it's why the figure of Death—personified as the driver of a carriage—has to stop for her. This highlights the fact that it is ultimately Death that chooses when we leave this world behind, not ourselves. On this reading, even someone who takes their own life is fated to do so.
The figure of Death takes the speaker on a smooth and pleasant journey from this world into the next, from the here and now into the realm of the eternal. The longer the poem goes on, the more ethereal the speaker becomes. Slowly but surely, she's turning into a spirit as Death's carriage takes her across the threshold into eternity.
https://www.poetryfoundation.org/poems/47652/because-i-could-not-stop-for-death-479