He's hardly the only critic capitalism has ever had, but he is far and away the most well known (I hadn't heard of Engels even until I got to college). What is it about him or his ideas (or the way that either of those have been branded) that make his name so exclusively synonymous with alternatives to capitalism?
Marx wasn't just a critic of capitalism, he was an economist and philosopher who created an analysis which seemed to identify fundamental flaws in the capitalist system and articulated a theory of history which offered a way out of the present state. He explained why capitalism produced stark inequality, why markets failed periodically and why class relationships changed over history. His work offered revolutionary movements and reformers alike a rational prescription for change. The Communist Manifesto is just a basic introduction. His work also allows a lot of subtle development and rethinking by later socialists. Even if you are not a Marxist (and I am not) it is a very useful world view for analysing history.
Mainly because he is one of the writers of the Communist Manifesto, leading to the only global political movement to successfully mount a major challenge to capitalism.