By Orbital Team
A launch window exists because a rocket cannot cheaply turn its orbital plane after launch. The plane of any orbit is fixed by the position and velocity of the vehicle at the moment its engines stop providing thrust, and changing that plane later costs enormous amounts of propellant.
For a rocket targeting a specific orbital plane, such as one that must rendezvous with the International Space Station, the launch site has to be directly under that plane at liftoff. Because the Earth rotates and the target orbit's plane stays fixed in inertial space, that alignment only happens once or twice a day, for a window that can be as short as a few seconds.
Geostationary transfer launches have more flexibility since the final orbit is equatorial and any longitude works, but they are still constrained by range safety, ground station coverage during the coast phase, and thermal limits on the payload while it waits inside the fairing.
Weather adds another layer entirely separate from orbital mechanics: upper level wind shear, the risk of triggered lightning from the rocket's own exhaust plume, and cloud cover that would prevent range safety officers from tracking the vehicle visually if telemetry were lost.
When a launch scrubs with the countdown clock reading zero, it is rarely a single failure. It is usually one constraint among a dozen, from plane alignment to a boat inside the exclusion zone, snapping the window shut a few seconds before it would have opened wide enough to fly.