To visualize one of the constraints (the orbit inclination) -- imagine holding up a hoop around a globe, with the hoop angled at 51.6 degrees. Now start spinning the globe until Florida intersects with the hoop. That is when it has to launch, to be going in the same direction as that hoop.
Spin the globe a bit further, and now you can see that Florida no longer intersects with that hoop. If you were to try to launch at that point, your trajectory would form another ring that intersects with the ISS's orbit, but at a different angle. So it would take quite a bit more propellant to change direction once in space.
Now, you also have to launch so that you intercept the ISS when it is at the proper position on that hoop. If, when you reach the hoop the ISS is half way around the world, you won't get to it. (Technically, you can spend some time at a lower orbit, then raise your orbit at the right time, or adjust the amount of time it takes to get to the right height, but again that messes with the timing of everything and fuel usage).
> Now, you also have to launch so that you intercept the ISS when it is at the proper position on that hoop. If, when you reach the hoop the ISS is half way around the world, you won't get to it. (Technically, you can spend some time at a lower orbit, then raise your orbit at the right time, or adjust the amount of time it takes to get to the right height, but again that messes with the timing of everything and fuel usage).
This is normal, actually. It's part of why it normally takes several days from launch to ISS rendezvous. (Another part is that the trajectory is very carefully calculated to not leave the spacecraft on an intercept course with the ISS if anything breaks) And it takes essentially no fuel - 100m/s of closing rate will let you catch up from halfway around the world in 2-3 days.
This makes sense. If you aren't in a huge rush to reach your destination and can spend 2-3 days to let your orbit match the ISS. If launch windows were weeks or months apart, a couple days of catch-up orbiting might be okay.
Though, if you can spend that same amount of time sitting on the ground and get there at roughly the same time, you'd waste less resources sustaining the crew in orbit, as well as reducing the "Ugh, are we there yet??" factor by a decent amount.
Spin the globe a bit further, and now you can see that Florida no longer intersects with that hoop. If you were to try to launch at that point, your trajectory would form another ring that intersects with the ISS's orbit, but at a different angle. So it would take quite a bit more propellant to change direction once in space.
Now, you also have to launch so that you intercept the ISS when it is at the proper position on that hoop. If, when you reach the hoop the ISS is half way around the world, you won't get to it. (Technically, you can spend some time at a lower orbit, then raise your orbit at the right time, or adjust the amount of time it takes to get to the right height, but again that messes with the timing of everything and fuel usage).