The Polo Grounds' end was somewhat anticlimactic, especially compared to other "Jewel Box" parks.
Part of the problem was that the stadium was not well maintained from the late 1940s onward; while the baseball Giants owned the stadium, they did not own the parcel on which it stood.
Moreover, the neighborhood around the stadium had deteriorated by the early 1950s.
All of this combined to severely suppress ticket sales, even when the Giants played well.
In 1954, for example, the baseball Giants only drew 1.1 million fans (compared to over two million for the Milwaukee Braves) even as they won the World Series.

The football Giants left for Yankee Stadium across the Harlem River following the 1955 NFL season, and the baseball Giants' disastrous 1956 season (most of which they spent in last place before a late-season surge moved them up to sixth) caused a further drag on ticket sales.
The Giants' 1956 attendance was less than half of the figure for the Giants' World Series-winning 1954 season.
This hit Giants owner Horace Stoneham particularly hard.
He was not nearly as wealthy as his fellow owners; the Giants were his sole source of income.
As such, the departure of the football Giants and the baseball Giants' increasingly meager ticket sales left him with little to no money for stadium upkeep.

Frustrated with the obsolescence and increasing dilapidation of the Polo Grounds, Stoneham seriously considered having the Giants become tenants of the Yankees in the Bronx.
He also considered moving to a proposed stadium that would have been owned by the city.
  However, when both of those plans fizzled, the Giants announced on August 19, 1957, that they would move following that season, after nearly three-quarters of a century, to San Francisco, California, following the Dodgers to the West Coast.
The Giants had won five World Series in the Polo Grounds.
The Polo Grounds' end was somewhat anticlimactic, especially compared to other "Jewel Box" parks. Part of the problem was that the stadium was not well maintained from the late 1940s onward; while the baseball Giants owned the stadium, they did not own the parcel on which it stood. Moreover, the neighborhood around the stadium had deteriorated by the early 1950s. All of this combined to severely suppress ticket sales, even when the Giants played well. In 1954, for example, the baseball Giants only drew 1.1 million fans (compared to over two million for the Milwaukee Braves) even as they won the World Series.
The football Giants left for Yankee Stadium across the Harlem River following the 1955 NFL season, and the baseball Giants' disastrous 1956 season (most of which they spent in last place before a late-season surge moved them up to sixth) caused a further drag on ticket sales. The Giants' 1956 attendance was less than half of the figure for the Giants' World Series-winning 1954 season. This hit Giants owner Horace Stoneham particularly hard. He was not nearly as wealthy as his fellow owners; the Giants were his sole source of income. As such, the departure of the football Giants and the baseball Giants' increasingly meager ticket sales left him with little to no money for stadium upkeep.
Frustrated with the obsolescence and increasing dilapidation of the Polo Grounds, Stoneham seriously considered having the Giants become tenants of the Yankees in the Bronx. He also considered moving to a proposed stadium that would have been owned by the city.   However, when both of those plans fizzled, the Giants announced on August 19, 1957, that they would move following that season, after nearly three-quarters of a century, to San Francisco, California, following the Dodgers to the West Coast. The Giants had won five World Series in the Polo Grounds.
Polo Grounds