In fairness, there are very serious institutional barriers to Bernie getting any of his policies enacted. I'm not sure what the answer is- clearly electoral politics alone will be insufficient to accomplish progressive change. However, the means of progressive change in the past - i.e. labor unions, viable communist/socialist organizations, and civilly disobedient mass movements - all seem to have either declined drastically over the past 2/3 of a century or else have been specific and complex sociopolitical phenomena that implies a lack of major changes coming soon.

Both the Democratic and Republican Parties are parties of big business, that would generally take political actions in favor of those businesses. The progressive wing of the Democratic Party is notably weak, Sanders campaign aside. America, of course, is not alone in this general dilemma - the British Labour Party, French Socialist Party, and other historically socialist and social democratic parties have followed the political right in austerity reforms, privatization, and other neoliberal actions.

Even if Bernie Sanders were to sweep into office with Democrats occupying enough offices in Congress to match their 2009-2010 supermajority, the actual history of that supermajority offers a further warning. The healthcare reform that became the Affordable Care Act was directed away from a public option, and the Supreme Court removed its most potent inequality-reducing aspects in a ruling not long after. In the highly improbable best case scenario, what we can get with Bernie is political compromise that will still leave you with your student loan debt, healthcare bills, and declining wages.

Maybe the question you should be asking is not, "Why do so many people say Bernie can't change anything," but instead, "How can we rebuild an effective progressive/radical movement in the US?"