I kind of answered this here: https://www.reddit.com/r/AskHistorians/comments/627my2/z/dfkv07s

Essentially, the English did manage to stave off a Viking conquest of England for over 150 years, and had a sophisticated network of civil defence, fortifications and fleets organised at a county level. In fact, by the 950s, the entireity of the Danelaw had been recaptured and integrated back into the politcal unity of a Wessex-led Kingdom of England. The Anglo-Saxon Chronicle tells us that during the reign of Edgar, no invasion or raiding forces successfully made it to English shores at all. Indeed, he seems to have been called Edgar the Peaceful not because he oversaw a lack of conflict, but because he was so militarily successful that England remained peaceful internally.

Where things go wrong for England is in the 980s onwards during the reign of Æthelred II, the "Unready". Æthelred took the throne at a young age after his more popular half-brother was murdered, and was politically very weak. A resurgence in Viking activity was handled particularly poorly, with the St. Brice's Day Massacre of 1002 as an attempted ethnic cleansing of Anglo-Scandinavians in the former Danelaw merely serving to alienate much of the population, sow wide discontentment and provoke international condemnation. Initially, Æthelred reinstates the Danegeld to pay off the Vikings, which hadn't been levied for over a century before. This, however, is levied as such a heavy tax that it turns great numbers of the nobility and bishops against him.

Æthelred's rule politically divided the country and the church, and he was very ineffective at controlling his nobility. This meant that disagreements often blossomed into full feuds and betrayals, and he allowed the previously effective defensive network to fall into disarray. In 1013-1015, Cnut's army is able to wreak havoc across the south of England, for example, because disagreement between the Ealdormen of Dorset and Hampshire had been allowed to deteriorate to the extent that the Dorset fyrd refused to come to the aid of the Hampshire garrisons, and in return the Hampshire fyrd refused to aid Dorset the following year. Subsequently, Cnut was able to fight both forces separately and defeat them, instead of being both outmanouevred and outnumbered as he would have been had he fought both forces combined had the burghal network functioned as designed.

Æthelred also makes the mistake of continuing to trust nobles who have betrayed him multiple times. One particular noble changes sides mid-battle along with his entire fleet, so Æthelred has his son blinded as punishment for his disloyalty, but still keeps that nobleman in a position of power. He's then entirely shocked when a year later, the same nobleman fails to turn up with his promised army to support Æthelred's campaign against the Vikings and he has to sue for peace. A planned grand campaign against the Hiberno-Norse on the Isle of Man designed to restore prestige and confidence in his rule has to be cancelled because simply none of the other nobles turn up to support the royal fleet.

It is worth mentioning, however, that despite all the turmoil of his rule, Æthelred rules for almost 30 years, and the English still win many victories over the Vikings until the political situation deteriorates too far.

TL:DR: The English do manage to hold off the Vikings for a very long time, but then things go completely to peices politically, and Cnut is well placed to take advantage of the massive internal strife within England which prevents an effective defence being mounted.