In short, they weren't. The Vikings never conquered England until 1016, and from the 870s until the 950s were on the back foot, with the areas they had taken being steadily retaken by English forces. During the reign of King Edgar (959-975), supposedly no foreign forces successfully reached England's shores.

In the 850s-870s, the Vikings won battles by out-manouvring the English, striking their armies piecemeal before they could unite, and withdrawing from battle if they did. In 854, a Viking force defeated a Mercian army before it could unite with a West Saxon army marching to join it, but were then subsequently defeated by that West Saxon force when it did arrive. They captures swathes of territory largely through surprise attacks, catching garrisons off-guard.

From the 870s onwards, King Alfred built a chain of fortified sites called burhs across England at prominent road junctions, river crossings, landing sites etc, which allowed the English to block Viking movements and rapidly deploy forces to cut them off and defeat them. This system was highly effective and was replicated by Alfred's children as they consolidated and recaptured English territory in the Midlands and the Danelaw.

The successful Viking invasion of England was led by Cnut in 1016, but this was a culmination of almost 20 years of periodic warfare and raiding which had seen victories on both sides. Cnut was able to take advantage of massive political instability in England at the time under the reign of Æthelred II, the "unready", who spent almost as much time arguing with his nobility as he did fighting the Vikings. The national defensive network collapsed as noblemen betrayed each other and the king, refusing to send troops to support the royal army, or refusing to send garrison forces to support other defensive sites. This meant, for example, that when Cnut raided Hampshire, Dorset didn't send the reaction forces necessary to support the local militias and trap Cnut's forces, so in return, no Hampshire troops came to Dorset's aid in 1015, meaning Cnut could essentially raid unimpeded. Some nobles supported Cnut because they disliked Æthelred, or in rebellion against his excessive taxation.

Whitelock, The Anglo-Saxon Chronicle

Roach, Æthelred the Unready

Stenton, Anglo-Saxon England

Lavelle, The Danes in Wessex