There are a number of possible answers to the question of why history has not turned out the way that Marx predicted it would. The answer that a person gives to this question depends to a great degree on the person’s political leanings. Let us examine two different ways in which to answer this question.
First, a person who is dedicated to the idea of capitalism (a conservative) will say that history did not turn out as Marx predicted because Marx was wrong about what people wanted. Marx argued that history was made up of a series of class struggles. He believed that workers in a capitalist economy would feel oppressed and would want to rise up against the people for whom they worked. Conservatives would say that he was wrong. They would say that workers do not feel rivalry towards the rich but that, instead, they want to be like the rich. Therefore, workers do not rebel against the rich but rather work hard so they can become rich themselves. A conservative would also say that Marx did not understand the degree to which workers in a capitalist economy had upward mobility. Because they could climb the economic ladder, they did not develop a sense of class consciousness and did not want to rebel against the rich.
A convinced Marxist would take a rather opposite view. This person would say that history has simply not YET turned out as Marx predicted. Marx did not predict when capitalism would fall and communism take its place. To a convinced Marxist, capitalist governments have been staving off their inevitable fall by doing things providing welfare states to their workers. They have also been using people in other countries as their menial workers, thus exploiting foreigners while keeping their own people happy. Finally, a Marxist might argue that working people are still being mystified by the upper classes who encourage them to unite on racial, religious, national, or other lines. Because the workers are being tricked, they do not develop a class consciousness and therefore do not rebel.
Thus, we can see that it is possible to answer this question in ways that are diametrically opposed to one another. There are also intermediate answers that can be given. For example, a person might say that upward mobility helps prevent workers from wanting to rebel while also believing that the welfare state has moderated the ills of capitalism and made a communist revolution less likely. A person might also point out that the world’s economic system has changed dramatically and that there is much less of a split now between “workers” and “capitalists” since so many workers are in white collar jobs that are unlike the factory jobs of Marx’s day.
The question of what Marx got right is also one whose answer is determined largely by one’s political values. A conservative would say that Marx got very little right. A Marxist would say Marx got most things right. My own view is that Marx was right when he said that capitalism causes tension between workers and their bosses. However, I believe that he was wrong about the degree of tension that would arise and about its consequences.
It is one thing to analyze what exists in the present and quite another to predict what is going to happen in the future. Diagnosis and prescription are two different functions. Many people can see what is currently happening, but nobody can foresee and foretell what is going to happen. If anyone could do that he could make a fortune. Marx certainly wasn't making a fortune in England; he was living on charity. Marx was a deep thinker and his analysis of capitalism was impressive. That did not qualify him, however, to be a prophet. He thought that communism would develop in the most highly industrialized country, which happened to be Britain at the time he was writing. Instead it developed in one of the least developed countries, Russia. This occurred because of the influence of Lenin, who apparently thought he could accept as much of Marx as was useful for his purposes but that he could make some alterations to suit the Russian Revolution. It turned out that the Russian people were hardly ready for such radical changes as Marx had predicted--so it took people like Stalin to try to make them ready. All of this was only a small part of what Marx could not have foreseen. George Orwell deals with the subject pretty thoroughly in his allegorical work Animal Farm.
While some might say that things have not turned out as predicted yet but will do so, perhaps the most common answer is that Marxist ideology is too idealistic. While in theory it is nice, there are some problems with it which make it almost impossible to practice in real life.
Humans tend to be greedy and competitive. It is easy to see if you look around you. Humans will always want to be better than one another and so forcing everyone to be equal does not work. The welfare system solves this to an extent by ensuring everyone has a decent quality of life, even though not everyone is equal.
Another problem with the Marxist view is that by eliminating capital, workers have less of an incentive to actually work.