As I argued previously, reproductive variance was the first inequality—all other forms that matter to us, like income inequality, do so because they have historically been related to reproductive variance. Those with more resources, for instance, had more babies that survived to reproduce; when possible, those babies also tended to inherit their parents’ resources, starting the cycle anew. These chronic effects on reproductive success have imposed selection pressures on the human mind to compete optimally for resources.
As far as hot button issues go, income inequality is certainly having a moment. A worldwide, popular movement was spurred on by it, several Nobel-prized economists have damned it in no uncertain terms, Barack Obama made it a pillar of his presidency (though, on this, he may have begun to waver), and The New York Times has devoted a regular series to it.