One of the major industries in Cornwall was mining, in particular tin and copper. When the industry went into decline in the early part of the nineteenth century, it happened to coincide with an increased demand for coal as the industrial revolution was to a large extent powered by it. As a result many Cornish miners and their families moved either to coal mining regions of England and Wales, or abroad to work in mining towns in Australia, the US, South Africa and even Mexico. This is a significant cause of population decline as hundreds of thousands left Cornwall in the 19th century.

As for a port, Cornwall is (in coastal terms) between Bristol and Portsmouth which were two of the most important ports in English history. Plymouth also has an port, but crucially it didn't have the industrial capability or transport links needed to deal with imported goods from the empire, so never grew to be quite as important.

A good explanation by /u/V-X. As a point of detail, Cornwall may have peaked in the nineteenth century at about 360,000 residents. During the nineteenth century, an estimated 100,000 immigrated. These were largely miners - the famed Cousin Jacks, who were often first in line for mining jobs internationally. More men than women immigrated, but a fair number of women left as well.

Working with the US census records I found a Cornish couple in the 1880 US manuscript census who had several children with their places of birth listed. From eldest to youngest the children were born in Cornwall, Bolivia, California, and Nevada. These were residents of the International Mining Frontier, and traveling to the "next place" for work was a given in an industry that drew on nonrenewable resources.

Cornwall did retain a China clay mining industry, and its copper and tin mining did not completely end until the twentieth century. It was never a highly populated place being on the road to nowhere. Tourism - and now the attraction of Cornwall as a place for second homes/country retreats for the financially able - has caused shifts in its population.

Phillip Payton has written several books on the history of Cornwall and in particular on its immigration. I recommend anything that Phillip has written. He is the leading authority on the subject.

The decline in Cornwall's share of England's population is remarkably recent: numbers were growing along with the country as a whole until around 1840, peaking for a century in 1861 (copper production peaked in 1856, tin in 1871): the 2011 census recorded only 50% more people than 150 years earlier. Much of the interior is rugged and unsuited to dense settlement but it remains a popular retirement destination, particularly the less exposed southern coast (for those who can afford it).