Is it because Marx wrote more theory than Engels?
I dunno, Engels just seemed like a swell guy the way he supported Marx a lot. A true bestie.
Also does this count as a bad post because of the title? Sorry.
Engels was quite brilliant in his own analysis and writing by himself. It's why Marx sought him out. It's just Marx was well Marx. It's a big intellectual shadow to be under. Like Lenin and Stalin from a theory perspective, Stalin is good at writing and explaining complicated topics but Lenin is Lenin.
And Marx and Engels were pretty much in complete agreement, I forgot the line but there's one letter from Marx which is like unless specified assume Engels and I both think this in all my writing. And a lot of Marx's writing surviving is because of Engel, he pretty much wrote Capital Vol 2 and Vol 3 from Marx's notes (Brandon Sanderson style) and published a few other books of Marx's before and after his death.
They had met a few times before, but it was when Marx had read "The Condition of the Working Class in England" by Engels, that they became lifelong friends.
The Condition of the Working Class in England is actually a stunning and brilliant piece of epidemiological work, it's hard to believe it was published before John Snow (London Cholera map guy) was even a doctor
I love that letter that goes like
"Fred, I need another 50 quid for rent.
Sincerely, Karl"
EDIT:
Show
"Karl, you need to do something about your conditions."
"I am, I'm writing a whole manifesto about the material conditions"
"I meant how messy your living spa- actually nevermind"
I think Engels wrote a lot of good stuff, but he was essentially a bourgeois class traitor (Marx's family were well-off, but they worked. Engels' were literally just factory owners), which I dare suggest meant his writing, though totally sound, struggled to be as relatable and punchy without Marx's influence. Marx had a raw, powerful, and vaguely intimate way with words that I think simply captured people's minds and attentions better.
Mostly I am just prompted to show love for our Engels emojis
and the best one 
Some time after Engels father died, he sold the factory. With the money, he supported not just Marx and his family, but a large part of the underground/communist/German scene in London. He also speculated at the stock exchange and he must have been good at it, because when he died, he left more than he had started out with.
Also, while Marx said: "Je ne suis pas marxiste", Engels really was the first Marxist.
Engels wrote better introductory texts that are more readable (Principles of Communism, Socialism: Utopian and Scientific; Marx did deeper analysis.
