This is less a question of history and more a question of geography (and a little economics). The short answer is that England simply has terrain far better suited to supporting larger populations. Most of northern Scotland and central Wales are mountains and uplands which are not condusive to easy agriculture. Until the Industrial Revolution brought large numbers of people, similar areas of England, such as the Staffordshire moorlands to the south of the Peak District were also sparsely populated when compared to the South-East, and South of England.

Weather plays an important factor, especially in agriculture. Wales and Ireland are extremely wet, as is much of South West England. Scotland traditionally has long, bitter winters. By comparisson, somewhere like East Anglia is temperate and dry.

Other factors include economic viability. In the Early Medieval period, the south and east coasts were particularly densely settled partially because this was the area through which prosperous trade across the English channel was achieved. In the 10th century, Chester and Bristol developed into major population centres because of their burgeoning ports, which facilitated a growing trade across the Irish Sea.