I would suggest that the theme and purpose is to reconfigure the depiction of death. The poem presents death as a part of the life process. The theme might be calling upon individuals to reexamine their traditionally help conceptions of death and bring about the change required in order to fully understand death's role in highlighting life and the part of life that death is. The closing of the poem seems as if that the speaker has fully understood where life is and where death is in that scope, a voice from beyond, indicating how death is not something to be feared or something where one attempts to repudiate, but rather fully embrace as a constant companion.
I see you now have many answers, but since I've written it, I'll go ahead and add mine anyway...
The primary theme in "Because I Could Not Stop for Death" is death and eternity. A secondary theme is unpreparedness. This poem is a salute to the inexorability of death, to the dogged journey it traverses. There are more than one opinion as to whether Dickinson's poem speaks of Christian consolation or not. Some view it as devoid of religious or any other consolation and see it as an expose, as it were, on the continual presence of the companion Death. Such an understanding would have been unpopular in the end of the nineteenth century even though religious conformity had lost its hold on spiritual thought.
Dickinson illuminates the primary theme of death and eternity, of the inexorability of death, by placing Immortality as a passenger in the carriage and by describing the centuries of Death's journey as "shorter than a day." The theme of unpreparedness is illuminated by, for example, the "Gossamer" gown with "Tulle" "Tippet," which is a fine see-through silken gown with loosely woven silk netting for a shawl.
I have a third interpretation of the poem from where I stand. To me, the poem "Because I Could Not Stop For Death" by Emily Dickinson is a lot about control issues. By all accounts, the poet herself was reputed to be more than a little 'buttoned-up' and I think that the somewhat humorous or irreverent tone is selp-deprecatory and self-mocking. It is as if the poet accepts her self-controlling (and even a little repressed) personality, but knows she will one day have one thing that will control her whether she likes it or not. Death comes to all of us, no matter how much control we think we have mastered over ourselves and life - we must relinquish control voluntarily or it will come for us and take it regardless at the end.
I have a slightly different view of Dickinson's purpose and theme in "Because I Could Not Stop for Death." The poetess was certainly not someone who lived life to the fullest, and while that does not always connect to the speaker ofa poem, in this case, I think that Dickinson's view of death is present. To me, her purpose is to discuss the universality of death and to stress the truth that no one escapes it. One can run from it or be too busy to think about it, but at some point, death "kindly stop[s]" for us.
Dickinson's whimsical tone suggests a bit of satire but also helps readers view death as just another part of the human journey or experience--similar to children playing or moving to a different home; see Stanza 5 for her description of a grave as a house.
In my opinion, this poem is about the need to live life as fully as you can while you are still alive -- so it is something of a carpe diem poem. It is saying that life is short and death is forever so live now while you can.
I think you can see this in how the journey that the speaker and Death take (along with Immortality) goes along beside things like schools that represent life. But then they get to the grave and the speaker realizes that that is where she will live for eternity -- a much longer thing than the short trip they have just made.