Capitalism creates massive amounts of wealth but concentrates it at the top among the "wealth producers" who gain a disproportionate share of the wealth created while creating negative externalities that harm society. Socialism in its various forms (ranging from communism to social democracy/modern liberalism) seeks to rectify these imbalances for the greater good. The issue isn't the theory, the issue is the practice. Socialist movements tend to run on a dangerous combo of hope, idealism and righteous anger, and that often precludes the movements for acting in their own best interest to create positive change. Socialist ideals have advanced the farthest by grass roots organizing, consistent, conscious political action, consensus and coalition building and exploiting divisions between the more far-sighted and moral members of the 1% (think Bill Gates, FDR, John Mackey, etc) versus the immoral, mean, sociopathic members (Trump, Koch Brothers, the Bushes etc etc). Doing those things requires compromise and sometimes having to table certain things you want (for example if you need benevolent rich people to win then you have to temper your demands for wealth redistribution, but in return you get progress in other areas and more institutional power), but the lefties you see around the Bernie campaign are not inclined to do that for various reasons, they'd rather break everything because the system is unjust. And in many ways it is,but that path is dangerous because at best, you'll accomplish nothing and alienate voters who value stability and don't want to risk current incremental progress on a massive upheaval, at worst, the massive upheaval will hurt vulnerable people.
Anyway I rambled a lot but the the main point is that vulgar socialism is unproductive and alienating.
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