Well, if there's one word that appears in this poem more times than any other, it's "alone." It makes sense (mathematically speaking) that the mode for the poem should also be its title.
If we're doing more than counting words, though, "Alone" starts to become a slightly problematic title. Just who is it that's alone? The speaker? The reader? The community at large?
The answer to all of those questions is "Yes." Just about everyone this poem discusses is alone. (Yes, that means you, too.) And we're not talking "Hooray! I ditched my baby brother and FINALLY have a few free minutes to myself" alone. We're talking "I'm about to have an existential crisis because my life sucks" alone. There's a big difference.
Maybe that's why Angelou doesn't entitle the poem "I am alone" or "We're all alone." It'd be too depressing to pick up. So, she leaves the title ambiguous. Sure enough, it sucks us in before we even realize that we are the real subjects of the poem. Tricky, eh?