There are fifty states in the United States and a bunch of territories. The States each get a star – 50 is a nice number to represent in a symmetrical pattern. The territories would just mess things up. So the flag has stuck to 50 stars since the Kingdom of Hawaii and Alaska joined in 1959. Think of all the old flags with 48 stars, and such.
Note, you can stop extrapolating the flag back to the original 13 colonies to figure out what the flag used to look like. It had a circle of stars in addition to the stripes.