In "Aner Clute" from Spoon River Anthology, what is the real reason she ends up wasting her life going from bar to bar?

She ends up "wasting her life," as you put it, because of one error in judgment that marks her for life.
In the poem, she claims that people asked her the same question you do:

Over and over they used to ask me...
How I happened to lead the life,
And what was the start of it.

Her answer is true to life, but not the real reason why she "[leads] the life":

Well, I told them a silk dress,
And a promise of marriage from a rich man--

This is the story that people expect to hear: a materialistic young woman "loses her virtue" to a rich man who makes "a promise of marriage," but does not come through.
It was, instead, the unfair judgment of others, and their need to mark her as a woman of ill-repute, that make it impossible for her to lead a normal life. She uses the analogy of a boy who steals an apple. Instead of getting sympathy and a brief admonishment, he becomes marked as a thief by every member of society. As a result, he cannot find work and decides to become what everyone believes him to be anyway.
Aner Clute is the same: because everyone wishes to believe that she is a tramp, she becomes one. Her identity is marked by one faulty act committed in her youth.