By the start of the Warring States period, regional leaders in China undertook large local engineering projects, like irrigation systems. Why is this considered evidence that the power of regional governments was growing at the expense of the Zhou central government?

It is evidence that local leaders controlled both the engineering expertise and the tribute labor that the Zhou government had once controlled.

Explanation:

Before the Warring States period, the local leaders were the one in control of both the engineering expertise and the tribute labor. This meant that the regional government, though present at that time, did not have sufficient power to control these. However, during he Warring States period, they began to take large local engineering projects.