The future USA changes, so it gets a new flag
In order:
The NUSA - Cyberpunk 2077
After the Collapse in the 90’s, there was basically a full reorganization of the US, and it basically became a vassal of Militech. Several states seceded and some were abandoned entirely due to the corporate wars and ecological disaster. The new flag represents the updated number of states and functionally represents the sovereignty of Militech.
Prewar America - Fallout
In 1969 the US decided to reorganize from 50 states to 13 commonwealths meant to group regions together by similar interest. The new flag has 13 stars to reflect the 13 commonwealths.
Postwar America - The Handmaid’s Tale
Following the successful coup and takeover of the continental US by the Christian extremist group Sons of Jacob, the only two US states remaining under American control are Alaska and Hawaii. The remnants of the US made a new flag with two filled in stars and 48 outlined stars, both recognizing the fact that officially there are only two states left, but they have only recognized the lower 48 as occupied rather than seceded or given up.
Victorious America - Watchmen
America wins the Vietnam War, then annexes Vietnam as the 51st state. The new flag has 51 stars to reflect the update.