Hello, OP! This question has been asked a few times before on AskHistorians, so I'll direct you to some links that might already answer your question.
https://www.reddit.com/r/AskHistorians/comments/33zbod/how_come_marx_has_overshadowed_engels/
https://www.reddit.com/r/AskHistorians/comments/2dgfee/friedrich_engels_and_communism/
I think the points that all the responses to these posts emphasize are broadly similar: firstly, Marx was more involved in revolutionary activity, including the First International; and secondly, that Engels acknowledged Marx as the source of many of the essential ideas of historical materialism and other aspects of what we call Marxism today. This isn't to say that Engels wasn't a considerable theorist and writer in his own right. Plenty of Marxists today still find lots of value in Socialism: Utopian and Scientific and Anti-Dühring, the latter of which had the purpose of advancing Marxism (as opposed to other strands of socialism) within the Social Democratic Party of Germany at the time.
Furthermore, Engels played a key role in the popularization and spread of Marx's ideas after his death. Besides his role in the publication of the later volumes of Capital, Engels's texts also introduced Marxism in a clear, easy-to-understand way to the wider public. As Robert Tucker writes,
one of Engels' greatest services consisted in the systematizing and popularizing of Marxism. Marx was the great system-builder, yet he never managed in the writings published during his lifetime to set forth the fundamentals of the system in easily understandable terms in an essay or short book; Capital was not bedtime reading for the masses. Engels made good this deficiency. He lacked the supremely trenchant style of Marx, who wrote as though his pen were dipped in molten anger. But Engels' beautifully clear and free-flowing prose was ideally suited to the popularizing work to which he applied himself.
The aforementioned Socialism: Utopian and Scientific might have been one of the most important texts in this regard. However, Engels's contributions towards Marxist theory were not insubstantial. For instance, much of what Marx and Engels wrote about nature or primitive society was influenced by Engels, although these definitely aren't the best-known or well-regarded parts of Marxist theory.
Another reason for the relative neglect of Engels in the West might be the Western Marxist focus on Marx's earlier works, such as his Theses on Feuerbach and The German Ideology (the former of which is the source of the famous quote about philosophers having to change the world rather than merely interpret it). This served to put focus on Marx's Hegelian influences, and also coincidentally minimized the influence of Engels, as Marx had not begun cooperating with Engels when he wrote those early works.
Gyorgy Lukacs played an important role in this in his focus on the Hegelian aspects/roots of Marxism in works like History and Class Consciousness and The Young Hegel. In particular, he launched a spirited defense of Hegel from a Marxist standpoint, and attempted to clear him of his reputation as a reactionary. For instance, in The Young Hegel, Lukacs claims that
even though his view of history represented a move to the right in comparison with his position in Jena, even though it involved him in a greater acceptance of German society as it was, the ‘owl of Minerva’ never became the carrion-crow of reaction under the Restoration.
This particular strand of Lukacs-inspired Hegelian-Marxist thought focusing on Marx's early work greatly influenced Marxist theorists outside the USSR in the early 20th century, including the first generation of Frankfurt School theorists (among them Theodor Adorno, who would later go on to attack Lukacs but acknowledged the significance of his early work on his intellectual development). As a consequence, the focus among Marxist philosophers and theorists outside the USSR was placed even more on Marx to the relative neglect of Engels.
Sources:
The Marx-Engels Reader, ed. Robert Tucker
History and Class Consciousness, Gyorgy Lukacs
The Young Hegel, Gyorgy Lukacs
The Dialectical Imagination, Martin Jay
Theses on Feuerbach, Karl Marx
Anti-Dühring, Friedrich Engels