As a broad generalization, liberals see income as a public good that is distributed, like crayons in a kindergarten class. If so-and-so didn’t get his or her fair share of income, it’s because someone or something — government, the system — didn’t distribute income properly. To the extent conservatives see income inequality as a problem, it is as an indication of more concrete problems. If the poor and middle class are falling behind the wealthy, it might be a sign of declining or stagnating wages or lackluster job creation. In other words, liberals tend to see income inequality as the disease, and conservatives tend to see it as a symptom.
Also, income inequality can be a benign symptom. If everyone is getting richer, who cares if the rich are getting richer faster? New York City’s inequality, for instance, is partly a function of the fact that it is so attractive to poor immigrants who start at the bottom of the ladder but with the ambition to climb it rapidly.