The answer is both complex and interesting.

For starters we need to understand one key concept: the Malthusian trap. The Malthusian trap is the concept that for most of human history, we have been subsistence farmers and the standard of living never rose for most humans above subsistence farming levels. We also know that the technological advances that happened before the industrial revolution mostly caused rises in population rather than in standard of living.

Once we understand that key concept, we have our real answer. The Victorians (as with most of Europe, the United States etc.) broke out of the Malthusian trap. This isn't exactly an exciting answer if you exclude why they managed to break out so I'll go into that in some detail.

Now, please note that the causes of the industrial revolution in Britain and abroad still remains a major source of debate between historians so I'll do my best to provide some differing opinions.

The first major cause for the industrial revolution for most historians is the major fall in the mid to late 17th and early 18th century in infant mortality. Mostly because by the mid 17th century the British were starting to abandon the old medical ways. Up until this point, medicine in Britain was still, frankly, medieval. The late 17th century would see the abandonment of many of the worse of what passed for medicine at the time (Bleedings, theory of humours, etc.) for a more scientific approach.

It should also be said that during this time, there was a marked increase in the quality and frequency of border controls. Prof. Paul Slack author of The Impact of Plague in Tudor and Stuart England points to this being a major factor in the reduction of frequency in instances of the Plague. Which means that diseases, in general, began to spread far less widely than they did in the past. This may have simply resulted in a rise in population but this time around it was considerably different because the rise in population coincided with major and numerous other advances.

Which brings me to our second cause. The massive increases in both farm labour (their productivity, to be clear) and land productivity. It all starts with the rise in the enclosure of farmland which reduces the amount of common land. The Crown and the Church had always been the two major land owners in Europe since the middle ages. In England, under Henry VIII, the Suppression of the Monasteries would free up massive swaths of land for free peasantry. It would gradually ramp up the enclosure and improvement of farmlands in comparison to their common land equivalent.

The Dutch also contributed a lot to the increase in productivity. the first major advancement used in Britain was the water-meadow. A system of pasture irrigation that isn't in use today but was a major advancement in the 16th-17th century in England. It allowed for the pasturing of livestock far earlier by keeping worked pastures in the spring damp, keeping frost off the ground and making grass grow several weeks earlier. This has no bearing on the question but I still think it's a fun little fact, men who took care and maintained water-meadows were called drowners. Not sure where this could've fit into my explanation so I'll just put it in here...

Agriculture during the period leading up to and during the industrial revolution is also marked by the introduction of scientific selective breeding. It saw the notable creation of the Dishley Leicester, the most productive sheep of it's era, and improvement of the Dishley Longhorn, which was the first beef cattle. Many contemporary sources talk of cattle weighing twice what they weighed after Bakewell's improvement of the breed, which is a considerable amount of extra productivity in cattle alone.

But let's return to the contributions of the Dutch for a moment, a notable engineer Cornelius Vermuyden, had by the end of the 17th century popularized the use of Dutch land maintenance methods. There are varying estimates about the effects of the land maintenance reform but there is no debating that it led to a substantial increase in the amount of usable arable land. Which meant that farms often increased in size.

The last truly farm-based improvement was the development of the Norfolk four-course crop rotation. Farmers switched from a three field system that resulted in large parts of farmland being left unproductive every season to one where there was very little need to leave any part of their land fallow. Where the British usually had fallow fields, they now put turnips or clovers on which to graze cattle, re-nitrate the soil and keep down weeds. Combined with the new breeds of cattle I've already talked about, this was a significant improvement.

So, this all meant that farms were far more productive far more easier than they used to be. Which meant that they could be worked by significantly less people. All at the time of a massive surge in population. The population of Britain went from around 9 million in 1800 to nearly 42 million in 1901. This was also the age where the British realized that they could transport goods inside the country far easier using industrial canals. So centralized production of goods was most definitely in.

So in short, as jobs left the countryside to the cities, so did people.

Sources: Harrison - The Common People, a History from the Norman Conquest to the Present

Clark - The British Industrial Revolution: An Economic Perspective

Kerridge - The Agricultural Revolution

Thank you for the gold kind stranger. This is my first gilded comment, I am very touched.

double gilded?! Thank you again kind stranger. That just made my day even better!

Simply put, one of the major causes was the Industrial Revolution.

Let's start with a tailor out in the countryside. All his life he's made shirts at X price for people on order, each shirt takes a certain amount of time to make by hand, and he makes a good living at it. With the Industrial Revolution, shirts are now sold at half that price (*numbers are not accurate, simplified for purposes of explanation) and the self-employed tailor cannot compete and needs to find additional alternative work for himself to support his family. Sometimes this additional support means that he needs to move himself and his family to a city where a factory has been established and work there for wages. Many people found themselves in these situations, where remaining in the countryside was just not an option.

Not all of the influx of population in the cities was people from the country. Many people from Ireland, fleeing the effects of the potato famine (1840s-early 1850s) sought work in factories in the cities.

I need to run to work now but can answer questions or elaborate more when I get back.

For sources/further reading... Asa Briggs's "Victorian Cities" is old (1965) but still relevant. Also see D. C. Coleman's "Myth, History, and the Industrial Revolution" (1992). The Victorian Web also has a pretty good page out there on the Industrial Revolution.

Increases in food production meant farmers had a hard time getting work when one farm could produce triple it's previous a amount with only 1/5th the amount of workers.

Cities were seen as where all the jobs were, and also there was a certain appeal to them in that they were considered where the modern world was advancing. During the mid 1800s, liberals especially were ecstatic about the industrial revolution and urbanization. Cities was where the world was changing, and many people were attracted to that.