I am no expert, but my understanding has been that there was this population explosion in Scandinavia, pushing many restless men out into the world to literally seek their fortunes. By the so-called 2nd viking age, they had pretty well mapped out the areas of their depredations which offered good alternative homes to settle down in.

England got the Danelaw, because the vikings came to settle and there was insufficient power in England to push them back out. It was not a campaign of conquest; it was colonization, sporadic and inevitable; much like the end of the Roman occupation encouraged the Saxons, Angles and Jutes to come to Britain and settle.

The same thing occurred on a massive scale here in North America: the European settlers pushed west and the natives simply got covered in the flood: there was no military conquest in mind, only increasing ones quality of life; and of course, many battles occurred as a result.