If you don't mind me simplifying your answer a bit... Imagine the orbit of the ISS like a tilted hula hoop around the Earth, with the Earth revolving inside it

If you don't mind me simplifying your answer a bit... Imagine the orbit of the ISS like a tilted hula hoop around the Earth, with the Earth revolving inside it. Because the Earth rotates once every 24 hours, any given spot on Earth will be directly underneath that hula hoop twice per day (once on either side of the hoop). When launching to the ISS, you need to launch when that orbit "hoop" is directly overhead. That's the bit that really matters, because you need to be in the same orbit. The ISS itself is almost certainly not overhead (it may be on the other side of the planet entirely), and so it will take some time to catch up to it.