Why do socialists often say that capitalism is a morally unjustifiable system?
The major reason that socialists tend to say this has to do with economic inequality. Capitalist systems have a tendency to lead to broad gaps between the rich and the poor. Socialists say that both the fact of these gaps and their origins are immoral.
The gap between rich and poor generally comes (to socialists) from the fact that the capitalists (those who own companies and such) take away the excess value that is created by the workers. In other words, the workers do all the hard work and the owners take the bulk of the profits. To socialists, this is immoral because the people who do the work do not benefit as much as those who only contribute money.
The existence of the wealth gap is also immoral. There is no reason, socialists say, that two people who work equally hard should find themselves in unequal economic circumstances. The same amount of effort should be rewarded with the same general level of material wealth.
So capitalism is immoral because it takes wealth from the workers and gives it to the capitalists, thus creating gaps in wealth between the classes.
Not only does it take from the workers, but capitalism as a system cannot function well when unemployment drops below around 6%, because it drives up labor costs and limits profits, among other things. What's more, the success of many capitalists depends on a large number of physical laborers and service industry workers, jobs which pay very little.
So, in short, capitalism requires that a significant percentage of society live in poverty in order to maintain the motivation of business opportunity and large scale profit. So a socialist would argue that any economic system that requires human misery in order to function is morally unjustifiable. They would also argue that such a system inevitably leads to political and social injustices.
According to David Hilfiker, a doctor who works with those in urban poverty (linked below):
An essential principle of the free-market system, then, is actually a formulation of injustice. The rich get whatever they want; the poor get nothing.