Zhou used a system where the local lords control their own lands and have too much power and the king has limited power despite being considered as the universal ruler. While it's an okay system where the kingdom was not large, it's very hard to do so since Han is much larger than Zhou. Qin system allow the emperor to control his vast land more directly without giving the local lords much power to begin with. Han has just been established and his generals control large armies. If Liu Bang used Zhou system then it wont be long before some of them decide to rebel. Even then, Liu Bang still used deep tricks to get rid of his generals and replace them with his relatives to further empower the power of the emperor. One of another important factors is that there are still a lot of people who are loyal to Han, Wei, Zhao and Qi. If Liu Bang used the old system then these people will leave Liu Bang and return to their own states instead. It will be one hell of a brain and power drain.

This actually had been shot down by Qin Shi Huang himself after uniting China. Some dudes also advised Qin Shi Huang to use the old system but he decided not to use Zhou system since he and majority of Qin advisors considered it as a failure system. Having local lords holding large armies is like asking them rebel against you.