If you do that, then you really have to address it in the legacy of Batman. Batman has to be so distraught over his parents death that he dresses up as a ridiculous "Bat" to fight crime that originates from poverty. But then Batman has to address that his parents weren't doing anything to end that poverty, when they very easily could have. Then Batman, being smart, apparently, would have to address the poverty, which, again, he could. End or massively alleviate that poverty, and move on, no longer emotionally, or physically needing to be Batman.
It's easier if they were good rich people, who's efforts were cut short. And he just fights costumed maniacs and the mob. Less ethical quandary involved.