Likely, it seems, because of the sheer quality of the food they ate.

This is a short article from the Institute for Brain and Behaviour

It certainly wasn't, no surprise, something to do with "calories".

... women typically consumed between 3,000 and 3,500 calories per day while men consumed 4,000 - 5,000 calories, with the navvies, who built the roads, canals and bridges that created the topology of modern Britain, hitting 6 or 7 thousand calories per day.
So what was it about their diet?

All in all, the mid-Victorian diet contained higher levels of vitamins, minerals and above all phyto-nutrients, than occur in today's over-refined, processed and nutrient-poor foods.

The Mid-Victorians did not eat a ‘super-Mediterranean’ diet because they were virtuous, or health-conscious. It’s obvious that the health benefits they enjoyed derived from the food choices they made, but food choices depend on food availability and on pricing, both of which were very different from today. Prior to 1900, fruit and vegetables were cheap, as they were mainly grown in allotments or gardens. With the rapid growth of the rail networks and the huge gains in agricultural productivity (the ‘agricultural revolution’), large amounts of fresh produce were funnelled into the cities, where the masses now lived. In London 4lb of freshly picked cherries or a large armful of watercress cost no more than a penny. A poor man's breakfast would have been two chunks of stoneground bread smeared with dripping, accompanied by a large bunch of watercress and often a paper of shrimp or oysters – a meal rich in fibre, protein, vitamins, minerals, omega 3 fatty acids and phytonutrients.
How has what we eat changed? · Food & nutrition · Institute for Food, Brain and Behaviour