Probably because 30 years of Reganomics has created the largest divide in wealth in America's history, while taking power away from the people and placing it with a select few.
Let's examine your argument a little deeper. I'm assuming you would not allege Obama was the catalyst behind the income inequality, being that his tenure has presided over the largest increase in per capita means-tested entitlements in America's history. Reaganomics is basically a trickle-down theory, while Obama famously wants to "spread the wealth around a little bit," as he told Joe the Plumber.
Which basically means Bernie supporters want him to continue Obama's economic theory, or even turn up the amperage a bit and increase it.
Obama took over right after a major crisis unlike one our country has seen in 70 years. He had stimulus funds of hundreds of billions of dollars and both houses of Congress to enact his grand vision. And how did that play out? 95% of all wealth gained since 2009 went to 1% of the people (Robert Reich's, Inequality for All).
Obama had eight years, and added trillions of dollars to our deficit, espousing the exact same economic theory as Bernie, and that was our result.
A heavy review of history shows only one barometric that has a direct relation to poverty and income equality and it's an inverse one. LBJ's Great Society and War on Poverty. Since his "War" began, the USA has spent $22,000,000,000,000 (far more than all other wars combined) yet the poverty rate is essentially an unchanged 14% (give or take a few minor tweaks depending on economic conditions). During that time, the poor have stayed poor and the rich have gotten vastly richer.
By aligning the facts of Obama presidency with the history of entitlements and Bernie's stated views on his economic theory, a supposition of Bernie merely keeping the poor in poverty, presiding over the soaring of the national deficit, and the rich getting vastly richer is frankly a foregone conclusion.