Why has the US flag changed 26 times?

Each year on July 4 the U.S. flag is changed if at least one state was admitted to the union since the previous July 4. The number of stars in the starfield of the new flag is equal to the new total number of states comprising the United States. Because the last time a territory was granted statehood was in August of 1959, the last time the U.S. flag was changed was July 4, 1960. As of July of 2006, the 50-star flag has been in use longer than any other U.S. flag (before July of 2006 the 48-star flag had been in use the longest), and as of July 2010 it is the only design that has been in use for over 50 years.