Why dind't Karl Marx like capitalism?

He didn’t. Marx thought that capitalism was the pinnacle of human social development at the time when he was writing. He believed it had done more to alleviate suffering and create possibilities than virtually any other development in social organization. This is the vital difference between Marxism and pre-Marxist or anti-Marxist left wing movements, which still exist today under a vague "anti-capitalist" umbrella. They oppose capitalism, Marx did not.

However, he also saw what he believed to be internal contradictions within capitalism that would eventually lead to its downfall. Most people today believe that his reasoning in this respect was wrong, even if they believe capitalism has internal contradictions. Marx thought the rate of profit would decline and eventually the working class would revolt as their wages were squeezed. His vision of socialism was a way to preserve the capital accumulation that is the great virtue of capitalism, but eliminate the profit motive.