More broadly, what were the economics associated with such estates? How did they make or lose money?
Two reasons: Changes in the economics of agriculture and trade and changes in taxation.
The "Corn laws" which protected cereal crops from foreign competition were repealed in 1846. This did not have an immediate impact on British Agriculture, but by the 1870s, bountiful American agriculture in the midwest, combined with railways and steamships providing cheap transportation to Britain, meant that grain prices started to fall.
This hurt the large estates which relied on growing grain. (Smalleer farmers, growing fruit, vegetables, dairy cattle, were not hurt badly, as these products could not keep well enough to be transported over long distances.)
The Income tax (which had been introduced to fund the Napoleonic Wars and then eliminated) was re-introduced in 1842. Over time the rate of this tax increased, and this hurt the large estates.
In 1853, Death Duties were extended to cover landed estates. Originally only on the annual value of the estate, not the capital value, but in 1894 the Death Duty on land was changed to a tax on the capital value.
The combination of lower returns from arable agriculture, income taxes, and death duties on land, combined to place continuous and increasing pressure on the viability of large estates through the last half of the 19th and the 20th centuries.
That is why so many of them struggled, were broken up, or disappeared.
Sources:
http://webarchive.nationalarchives.gov.uk/+/http://www.hmrc.gov.uk/history/taxhis2.htm