Essentially because Cnut’s actions as king were not particularly radical, but William’s totally transformed England. It would be nigh-on impossible to list all the ways in which early Norman rule changed the country, but here’s a few:

- Military – Before Hastings the English had never really used cavalry, and castles were still so new-fangled that there wasn’t yet a word for them in English. By contrast, the Normans made fantastic use of the cavalry charge during their invasion and started constructing what were essentially flat-pack forts that they brought with them from France even before Hastings. They later made extensive use of castles to pin down strategic areas, often demolishing huge areas of town centres to make way for them, changing the very landscape as well as the way war was waged.

- Religion – By 1100, almost every Church in England was razed or torn down and rebuilt in the continental style. English saints were reassessed and appropriated, and the clergy purged, with Wulfstan being the only English bishop left in his post a decade after the Conquest. They reformed the way spiritual pleas were dealt with too: they now had to be heard in special ecclesiastical courts appointed by (usually Norman) bishops, not in the usual hundred courts.

- Wealth – Wealth was radically redistributed towards the conquerors, all but eradicating the native English upper class in the process. By 1086 only four English lords remained, and the land that was once held by 4000 English thegns was now owned by just 200 Norman barons. In fact just ten men (all close to William) soon owned 20-25% of all land.

- Language – Prior to 1066 the English spoke Old English, a Germanic language. With the arrival of a French-speaking nobility which stayed in place for centuries, English absorbed a lot of latin-derived words. Interestingly, we can still see this divide between the French-speaking nobility and Old English-speaking subjects in our language today: names of animals (sheep, cows, deer) have Old English roots as they were named by peasant farmers but the meat upon which the nobility feasted (mutton, beef, venison) comes through French. Likewise, coarse swearwords tend to be Old English in origin, but the “medical” or “proper” term for them are almost always latin-based. This can also go the other way though, as Old English words tend to sound less aloof – think of the difference between a “hearty welcome” and a “cordial reception”.

- Violence – There were a series of rebellions in the wake of the Conquest, and one of them resulted in the Harrying of the North – in an effort to put down a rising, William rode to Yorkshire and employed scorched earth tactics which the north took decades to recover from. By some estimates, he reduced the value of the land by two thirds, and killed tens of thousands on the expedition.

- Politics – This Conquest brought England into a continental empire, and shifted the political centre of gravity to France. Even 150 years later, Richard Lionheart, popularly thought of as a quintessentially English king, only really goes to England to use it as a cash cow for his crusading. He was born in France and spoke French as a first language, as did many of his successors.

As I said, this is only scratching the surface of how the country changed: for many, the Conquest brought such profound change that they though the Normans to be a scourge from God. Anyway, hope this helps!

/u/SovyetskiiBoiyuz gives one view of the impact of the Norman Conquest, but for the last three or four decades, at least, an alternate view that rather downplays the impact that the Normans had on English law, society and government, and that stresses continuity over change, has been gaining ground among scholars of this period.

In 1066 And All That, Sellar and Yeatman famously asserted that the year of the Conquest was one of only two dates in English history that everyone could remember. Today, it may be the only one, and certainly it remains, in the popular conception, a major turning point – one characterised by the subordination of one people by another, and the introduction not merely of new rulers, but also of new ideas, new institutions, and a new – supposedly “feudal” – system of government. The Conquest certainly did result in an almost complete substitution of one ruling class by another; by the time of the Domesday inquest of 1086, the number of Anglo-Saxon nobles could be counted on the fingers of one hand.[1] It resulted in the replacement of Anglo-Saxon with Latin as the language of government. The underpinnings of the Norman system of government – the means by which patronage and protection were exchanged for service – was by no means an exact parallel to its Anglo-Saxon equivalent. And there is evidence that Norman influence penetrated deeply into Anglo-Saxon society relatively quickly, a change that can be seen in the rapid rise in the popularity of names such as William, Robert and Matilda.[2]

Yet this picture of rapid and extensive change is politically and historiographically charged. What Chibnall calls “the myth of the ‘Norman yoke’”, involving “the wholesale importation of Norman customs and institutions” [3] – a view common as early as the Civil War period, and one that endured well into the 20th century – served several important purposes. It excused Anglo-Saxon defeat by attributing it to overwhelming force; it glorified the survival against the odds of what were labelled essentially “English” qualities, such as love of personal freedom. And it neglects a good deal. It understates the wealth and the sophistication of the early 11th century Anglo-Saxon state – which was probably more wealthy and more sophisticated than any other in Europe west of the Byzantine Empire at this time, and which was certainly more so than the Duchy of Normandy. In popular accounts, it all too often also overstates the rapidity and the extent of the impact made by the new dynasty.

Domesday, our most detailed guide to what changed after Hastings, dates to 40 years after Hastings, and needs to be used with caution as a guide to the England of 1067 or even 1075. On closer examination, for example, the imposition of Norman rule outside William’s early strongholds in the south-east can be seen to have taken decades to achieve, and to have required the suppression of significant opposition in the south-western, eastern and – especially – northern extremes of the country. Imposition was, moreover, the task of a Norman ruling class that that is unlikely to have been more than around 10,000 strong in the immediate aftermath of Hastings, one which found itself in control of a nation of 1.5 million. Contemporary accounts stress how dangerous it could be for the members of this group to be abroad, away from the safety of the chains of castles they erected, unless they moved in large numbers. [4] And in a new Anglo-Norman state divided by the Channel, one in which the southern portion of the king’s lands was generally under greater threat than the northern one, and in which William himself, and his familia regis, spent three-quarters of the period after 1072 in France,[5] sweeping reform would in any case have been hard to impose. It is not surprising, in such circumstances, that as much use as possible was made of existing Saxon institutions; that as much trust as William dared was placed in surviving Saxon magnates (both lay and clerical); and that the change that did occur was neither immediate, nor necessarily widespread. The full effects of Hastings took years to be felt, and (in an age in which the much-excommunicated archbishop Stigand clung on in post as late as 1070, and the main Saxon tax, the geld, continued to be levied until 1162), the word that best sums up early Norman rule is surely “pragmatic.” For all these reasons, the initial impact of the Conquest on law, society and government were more limited than many historians have allowed, and it is easy to exaggerate its impact even in the longer term.

[Read my long answer with memes on my blog: https://asinusdocet.wordpress.com/2019/04/11/william-the-conquerer-turning-point/]

[Part 1]

I see many discussions on historical facts, but I think the best approach to answer this difficult question is with historiography: the writing of history. Also I'm pretty happy because I can provide some light on the subject. I hope you'll like it.

I read in J. Bellis’ monography (The Hundred YearsWar in Literature. 1337-1600, 2016*)* that “in 1754 David Hume held that ‘William had even entertained the difficult project of totally abolishing the English language’, persuaded by the fabrications of the chronicle of pseudo-Ingulf, not until 1826 demonstrated to be a fifteenth-century forgery not an eleventh-century original. […] In their emphases on the conquerors’ alleged attempts at total linguistic abolition, the emotive invective of the French conflict of the eighteenth and nineteenth century was surely rearing its head: Hume and Scott were (unwittingly, perhaps?) the Gloucester and Castleford of their generations.” (p. 28)

A few pages before, J. Bellis writes on Gloucester and Castleford’s works.

“Early in the thirteenth century, Lazamon described the Normans as ‘unnatural people’, the dominance of their language as ‘malicious practice’ and ‘an immoral act’. […] He represented conquest as a specific act of linguistic vandalism, an overwriting of the precious and precarious textual landscape that encoded British history.§ Such romanticised reconstructions became more exaggerated as the events became more distant. In the early fourteenth century, the Metrical Chronicle (historically attributed to Robert of Gloucester, although this is debated) inveighed against the fact that ‘the English people have been dragged down to the ground, on account of a false king, who had no right to the kingdom.’ […] Two whole centuries after 1066, it projected an image of the conquerors as an unassimilated, unwelcome group, and the English as a racial subset living amongst invaders. […] Thomas Castleford took the topos further, writing of the Conqueror, ‘he cleft the land of England from English blood’. […] He depicted the dominance of French in the legal system as a deliberate ploy to condemn the English in a language they could not understand.” (p. 21-22)

J. Bellis concludes:

“This romantic exaggeration, itself inherited from the later Middle Ages, was in turn inherited by criticism in the nineteenth and twentieth centuries. Edward Freeman (1876) held that ‘the tongue of the Norman conquerors … utterly displaced the national tongue’. He called ‘the abiding corruption of language’ ‘the one result of the Norman Conquest which has been purely evil’, lamenting that ‘the tongue which we brought with us from the elder England … has become for ever the spoil of the enemy’. Again, this reflect thirteenth- and fourteenth-century constructions (conducive to eighteenth- and nineteenth-century reconstructions), not the reality; but it neatly illustrates for just how long the depiction of ‘linguistic conquest’ went on gathering momentum.” (p. 28)

We can therefore trace back a long chain of texts that construct the 1066 Norman Conquest as a total disaster for England and the English language in particular. It all begins in the 13th century with Lazamon point of view on the matter, which is then inflated by Gloucester and Castleford’s writings in the 14th century. This leads to the writing of the pseudo-Ingulf chronicle in the 15th century, “that purported to be an eleventh-century monastic chronicle” (Bellis, 2016, 25), but that is nothing but a forgery and has been written to sustain the war propaganda against the French. That last text was taken very seriously however by Hume and Scott, who romantically exaggerated the impact of the Norman Conquest on the 11th century Anglo-Saxon England. Their point of view was then carried on by the honourable scholar that was Edward Freeman and the impact of the Norman Conquest has been discussed ever since.

On the other hand, there is no such historiographical debate, spreading through several centuries, about Knut’s conquest. Partly because, “as Pearsall hols, ‘Englishness had always been constructed’ negatively, ‘out of opposition to Frenchness’, which ‘had been the whetstone of English national sentiment from Norman times and … was always capable of being reinvoked’.” (Bellis, 2016, 64)