In a post this morning at Comment is Free, Peter Thompson asks 'how Marxism came to be the dominant theoretical apparatus of socialist thinking from the late 19th century onwards' and identifies its 'unique selling point' as follows:
As arrogant and dogmatic as it sometimes sounds to our ears now, Marx's USP was that his and Engels' approach to understanding history was the first to be based on truly scientific socioeconomic analysis. With a nod towards Darwin, Marx and Engels contended that their analysis of history was akin to a theory of evolution based on the concrete evidence of material facts. They argued that the theories of their rivals, the utopian communists and anarchists as well as the Hegelians and liberals, were based in idealist moral abstractions which dealt in notions of freedom, justice, fairness and equality in what they called the political superstructure of society, while theirs were based on an objective and scientific understanding of the real but largely invisible forces at work in the socioeconomic base.
Note that Thompson not only says that this is what Marx and Engels themselves saw as being distinctive about their approach; by laying it out in answer to the question he started out by asking, he implies that it is this feature of Marxism that also explains why it became dominant within socialist thought.
It is undoubtedly true that Marx regarded his theory's USP as being what Thompson says it was. However, I would question whether Marxism could have had the reach that it in the event achieved had it not been for the plain moral content of Marx's thinking. Though Marx often pooh-poohed the appeal to 'notions of freedom, justice, fairness and equality', he himself had recourse to these notions all the time; and it is unthinkable that Marxism could have had the wide appeal it did had the vision of a just, non-exploitative society in which 'the free development of each is the condition for the free development of all' not been at the heart of it.
The appeal of Marxism may well have been due to Marx's conjoining a moral critique of capitalist society and vision of an alternative future with the claim to have based his diagnostic ideas in scientific research about economy and society. But without the moral critique it is doubtful that Marx's outlook could have convinced so many. Having science putatively on your side may be reassuring, but not so much if what the science is supposed to show is either a matter of indifference or a cause for alarm. A crucial part of Marxism's appeal is the view that capitalist societies are pervasively unjust. As they indeed are.