The primary reasons for the decline of the English gentry were taxation, reformed voting laws, the industrial revolution and war, according to About.com. To a lesser degree, women's suffrage and the rise of divorce were also contributors.
For centuries, only the land-owning gentry could vote in England, which meant they could control the laws regarding land ownership. Naturally, these favored the wealthy. In the early part of the 19th century, however, voting law was reformed, expanding the voting population in England. This initiated the decline of the English gentry. The voting issue for the gentry was compounded by the industrial revolution, which saw the establishment of factories in urban areas and the migration of workers from the country into the city. The establishment of the railroad made the transportation of goods and people much faster, and farming declined.
By the early 20th century, land taxes were significantly higher than they had been in the past. Tenants who had migrated to the cities for factory jobs that paid better than farming were no longer available to pay rent, so many land owners could no longer afford to maintain their estates. As a result, they began selling off all or part of them. Additionally, World Wars I and II dealt significant blows to the British economy at large. Women also had the right to vote and own land. Since there were no laws restricting inheritance to sons, fortunes were more widely disbursed and divorce became more common.