The following three things help large corporations establish monopolies.
Big government. The bigger the government, the bigger the government contracts. (The more money there is to be made from government, the more corrupt businesses and politicians tend to be.)
Excessive regulation. (Some regulation is necessary. In China they have almost no regulation, allowing almost anything—including dangerous work practices and the theft of intellectual property. The guiding principle seems to be that if it benefits the CCP, it is allowed. In many ways—despite China's totalitarian government—China is the most laissez-faire economy in the world.)
The following video explains why big business in generally is behind excessive regulation in the West. (Watch from the 22nd minute. After clicking the video click “Watch this video on YouTube.”)
The final one is counterintuitive, but just as important as the other two.
When there are lots of small businesses, and many of them are growing, that is a sign of a healthy free market economy.