“In a room sit three great men, a king, a priest, and a rich man with his gold. Between them stands a sellsword, a little man of common birth and no great mind. Each of the great ones bids him slay the other two.
‘Do it,’ says the king, ‘for I am your lawful ruler.’
‘Do it,’ says the priest, ‘for I command you in the name of the gods.’
‘Do it,’ says the rich man, ‘and all this gold shall be yours.’
So tell me – who lives and who dies?”.

– Lord Varys, A Clash of Kings, ch. 3

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It primarily has to do with the conception of state-level authority, and where it – at least supposedly - lay.

Prior to the State of Qin’s total victory over the other warring states and its reformatting of the whole political calculus, political power was far more decentralized. The Zhou (and insofar as we can believe in their existence as per set down by Sima Qian, the Shang and Xia before them) inherited and operated a theocratic feudalism. The King of Zhou (and he was indeed a king (王), as the term for “emperor” had yet to be invented… but more on that later)… was, if not quite divine, then at least divine-adjacent, true enough. That status as high priest and communicator with the heavens and spirits, however, did not translate to wielding significant political or martial authority.

Rather, in the typical sense of “feudalism” (which I know is a word with a whole host of its own issues, but it conveys the point), actual secular authority over the various states of Zhou – outside of the central royal demesne itself – were given to particular “favorites” of the royal family, almost always themselves distant members of that same clan. They would rule in the king’s name, yes, but as Dukes (公), a title which – as its translation suggests – was an inherited position. This was significantly different from any and every position of the imperial era, since there on out, even at the highest pinnacles of power, each and every role and title in the empire was solely at the Emperor’s discretion and pleasure, could be revoked at any time, and resolutely was not automatically inheritable by the former occupant’s heir upon his death (in fact, such a practice was overwhelming frowned upon in almost every case). In any case, the Zhou Dukes functioned with near total autonomy within their own realms. This regional power was compounded by the Kings successively losing what little semblance of political significant they ever had from the 8th-7th c. BCE and the transition into the Spring and Autumn period of Eastern Zhou. By that point, the Zhou Kings were little more than a king on a chessboard – ceremonially powerful, as he who possessed – erm, I mean “protected” – the king was de facto more legitimate than one seen to be “attacking” him; yet they were to the very last manifestly helpless and powerless, subject to the wills and whims of the regional duke-cum-warlords who vied increasingly in the open for total control.

It’s worth noting that these states, far from being simply regions of a larger “whole” kingdom or nation, could be considered only distant “cousins” politically, socially, and even linguistically. They had markedly different governmental formulations – and throughout the Spring and Autumn and the Warring States Period itself, itinerant scholars would travel from place to place, seeking to convince one ruler or another that their teachings would be the “key” to reforming their own state into the version that would “win.” Monetary systems, systems of weights and measures, width of roads, even language and writing had tremendous variance between the states… really, some of the only things the denizens of these states “shared” was the nominal religious allegiance to the King of Zhou.

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OK, so let’s take that overview and flash forward to 221 BCE and compare it to the eventual victory of the Warring States, the 38-year-old King of Qin, Ying Zheng. The King of Qin (and you’ll notice by this point that the styling had changed – generations prior, in fact – since the Zhou Kings had long since worn out even their ceremonial usefulness, and Ying Zheng’s own great-grandfather, King Zhao of Qin, had stared down the King of Zhou in the 256 BC and said, “look at me, I am the king now.”)… anyways, he and his father before him had gone whole-hog on a radical system of reformation and state-building that sought to re-build Qin from the ground up – taking it from a borderland nowheresville and turning it into an unstoppable juggernaut. That system, called Legalism (法家), stressed the absolute inviolability of the sovereign and his laws, and demanded an absolute submission to a singular authority figure – obviously the king himself. In spite of its harshness and brutality, the results spoke for themselves (say what you will about autocracy, but damn if the trains don’t run on time) – in the span of a generation Qin had indeed remade itself into not just a player, but the meteorically-rising state all the others were increasingly worried about.

With Qin’s total domination victory in 221 over the last of the sputtering warring states, Qi, King Ying Zheng enacted his Legalistic laws and reforms over the whole of the conquered territories. This mandated uniformity in all things – weights, measures, axel-width, coinage, legal codes, orientation, family units, housing, bureaucracy, language, writing styles, and even thought itself. Legalist Qin had worked long and hard to overcome the other competing schools of thought of the Warring States – Confucianism, Yin Yangism, Mohism, Yin-Yangism, Logicism, just to name a few – and it wasn’t going to have them around mucking up the works with their mealy-mouthed garbage philosophy. They all had to go. The traditional telling is that Qin had the scholars buried alive (a rather common practice at the time for defeated foes) and their texts stacked and burned. This is likely a later apocryphal literary invention for the sake of narrative drama – no once can deny that burying your enemies alive while their lives’ works burn is pretty metal. In any event, it is clear that a wholesale purge of heterodox thought was swiftly initiated – so thorough, in fact, that of the so-called “Hundred Schools of Thought” that had been popular during the Warring States, we even know of only about a dozen – and of them, sometimes only their name, as referenced in a surviving text.

That the ruler of Qin – once merely one state among several, but now the whole territory – was something manifestly different and more than the rulers that had come before, none could argue. As such, the title of “king” seemed antiquated, ill-fitting, and archaic. After all, he was no ceremonial figurehead meekly consulting oracle bones, but the iron-fisted ruler of all, who directed millions by his imperial writ. A new position of such caliber and power demanded a new title. For that new title, Ying Zheng reached back to the very foundational legends of the primordial world – the mythical time of the Three Sovereigns (三皇, sān huáng) and the Five Sage-Kings (五帝, Wŭ Dì) - respectively, the godlike rulers of the beginning of human history, and the demigod quasi-mortal sage-kings who came after them. Ying Zheng therefore combined the two ancient titles into a new one befitting his status as the Absolute Thearch, combining the religious authority of a sage-king with the temporal and military power of a sovereign – a 皇帝, huángdì – typically translated as “Emperor.”The emperor was invested with all sovereign authority, and total command over every facet and part of the realm, at least in theory. In practice, of course, each and all would require armies of bureaucrats to oversee and assist in that rule – but they were ultimately servants of the emperor’s pleasure, rather than hereditary titles of nobility, or vested with any power in their own right. That is the substantive difference: the overhaul of where and in whom ultimate political authority was understood to reside.

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Sources:
Bodde, Derk. "The State and Empire of Ch'in" in The Cambridge History of China, vol. 1

Chang, Kwang-chih. “China on the Eve of the Historical period”, in The Cambridge History of Ancient China: From the Origins of Civilization to 221 B.C.
Lewis, Mark Edward. “Warring States: Political History” in The Cambridge History of Ancient China: From the Origins of Civilization to 221 B.C.
Shaughnessy, Edward L. “Western Zhou History” in The Cambridge History of Ancient China: From the Origins of Civilization to 221 B.C.

Soothill, William Edward, The Hall of Light: A Study of Early Chinese Kingship.