Marx wasn't even the proponent of a "system". And he was very critical in fact of utopian socialists, as well as people who called themselves "Marxists". The one thing he did was analyzing the exploitative character of Capitalism, nothing more, nothing less. He thought there would be a gigantic uprising of the workers worldwide, but that didn't really happen. Instead the cult of personality under Stalin and Lenin took over in Russia. And they were far from socialist too. Socialism can be summed up in a single sentence; ownership of the means of production by the workers. If there's a reference to private property in socialist literature, it is exactly that; the stuff that Capitalists own to create profit. It is not a reference to personal property, which is your stuff.
They went away from the source because the endgame in what Marx wrote is 1. Impratical and 2. Not a good thing (even though stalinism and all of those were a disaster too)
Plain and simple.
Karl Marx wasn't even outright critical of Capitalism, in fact he praised some of the aspects it generated, but only up until a certain point. Beyond a certain point there was just this system that was expanding for the sake of it, serving those who owned the means of production, at the expense of the working class. Today it is also obvious that this goes for the natural environment as well.
Another little capitalistic secret that no socialist likes to tell you about.
Capitalism improved the life of almost
everyone in the last 150-200 years after some adjustments. See how you ancestors lived in from 1600 until 1850, and from 1850 until now. It's undeniable that capitalism and industrial revolution generated technological advancements that was almost impossible to have under a centralized economic system. And after some time this things went down the maslow piramid of priorities for almost everyone. So now we can complaint about other important things like if the Last Jedi is a good or a bad movie instead of harvest/hunt our next meal.
Poverty felt more in the last 40 years than in every single period of human history.
You have to thing statistically. And guess what... unless you live in some REALLY fucked up country and/or in a really dire condition you are the "1%".
It seems to me that this almost phobic cynicism towards Socialism is a very American phenomenon, and I can't really blame them after many decades of fear mongering of so-called Socialism. And of course in the present day, when you have a lot of outrage news that equates Socialists with weirdos, SJWs, etc. There were people in America, in the past who rose up against Capitalists, the robber barons; and these people were no different than the Socialists that were in England, France, or Germany at the time. But dissatisfaction was dealt with by the rise of the police force. Many people died, not only under terrible working conditions, women as well as children, but also the people who rebelled against it.
Negative, this phobic cynism towards socialism happens in every country that experienced anything close to socialism. The problem is that almost everyone mistakes socialism with some sort of social capitalistic democracy (that... guess what... is unsustainable too)
Another socialism secret that no one likes to talk about and probably will fall like a bomb on SJW.
Socialism NEVER cared about minorities and enviromental issues.
This began as a trend only after URSS felt because anyone that lived under this shit wanted to buy a mcdonalds dollar menu in less than a minute and not wait for hours in line to buy a "fifht" grade piece of "meat" (when they have meat). So they had to refocus or disapear.
For the rest, see the second part of my answer,
Nowadays we live somewhat comfortable at the expense of the third-world, as well as China. Just check out the conditions these people live under, and then tell me that this Capitalistic, neoliberal system is in any way in favor of human prosperity, both material and non-material. But America, from what I have seen and read, is also in a terrible state; the great rise in homelessness, gentrification, the epidemic abuse of substances, the elderly, poor and disabled that get screwed over, the rise in suicides and violence towards others, the decline of a feeling of community, the overall hopelessness of the average person who tries to hold his head above the water in this neoliberal nightmare.
Negative again, captalism main goal wasn't do distribute wealth but it's undeniable that capitalism, free market and competition brought almost a century and a half of life of improvements never seen in human history to almost everyone.
That doesn't mean that we don't have to tackle the other issues. But is WAY EASIER to do that in this "neoliberal nightmare" than in any other system.
People who may not be in favor of this system have to live under it, and cannot currently escape it. They have to make use of certain means to spread awareness of the problems it creates. If you were a peasant in feudal times didn't mean you had to like living under these conditions. There are plenty of films, music, literature, and indeed even video games that expose the system we live under. Take the cyberpunk genre for example. How many times do we complain, on this very forum, about the exploitation of the consumer per week? Well, that is because of this very system that is in place people. We all complain, but not everybody is willing to put their finger on the problem. Even Oddworld's creator, Lorne Lanning, said that Capitalism has been detrimental to gaming. Why? Because it needs growth, just like a cancer.
I don't even consider myself a socialist or communist. In fact, I'm leaning towards Anarchism. But I have to give credit to Karl Marx, as he was right about a lot of things, for instance that economic upheavals are endemic in a Capitalist society.
You may not like to live under it, but it's undeniable and wonderful that today you can talk about it at a no cost internet forum in you cellphone, without having to deal with a Stazi like police/ Party hunting you
And as i said before, Marx was right as a broken clock twice a day
Every single competent economic student from a good orthodoxic economic school can pinpoint out every single fallacy and implode Das Capital. This isn't even in debate.