Your general impression that the geography of the Yellow River valley presented few obstacles to unification is accurate. However this in and of itself doesn't mean that numerous small states couldn't coexist in the region for a time. The first states in the Yellow River basin were essentially city-states- one of the names for China in use during the classical period was Wanguo 萬國, literally "the Ten Thousand States"- and a unitary Chinese Empire could only have emerged through a gradual social, political and military process of these small states coalescing into larger and more centralized ones.

You may also be under the impression that a once-unified Western Zhou state fragmented during the Spring and Autumn period and that Zheng, Cao, Cai, Wey and the rest were successors or breakaways. In fact these states were autonomous from Zhou government from the very beginning. The Spring and Autumn period was not about one large state breaking apart into smaller ones, but rather about the breakdown of a stable multi-state power structure with one clear hegemon and the emergence of a new, unstable power structure marked by intense conflict among numerous would-be hegemons. The early Zhou kings had been powerful and prestigious enough to keep the peace between the feudal lords (Zhuhou 诸侯) even without exercising direct rule over them, while the later kings did not and conflict between the lords escalated as a result.

Another thing to note here is that while there were many lords in the Central Plains, during the Spring and Autumn period only one of them held the title of "King" (Wang 王)- the King of Zhou. In classical Chinese thought, there was only one legitimate King in all of the civilized world, and this title had passed from the mythical founders of Chinese civilization, through the Xia and Shang royal lines, and then on to the royal house of Zhou. The feudal lords of the Central Plains states, including Zheng, Qi, Yan and others, held lower titles, such as "Duke" (Gong 公) or "Marquis" (Hou 候). It was only later, in the Warring States period, that most of the remaining feudal lords declared themselves Kings in their own right, formally ending their fealty to the Zhou dynasty.