Oh, this reminds me of high school, when I asked a physicist my mother dating about the same idea. He worked for the Rand corporation and was overall pro-nuclear. Still, He considered for a second and did his best to explain...
What I remember of his explanation was the point that if you launch a payload of material into space, you have essentially placed that payload in an elliptical orbital path, one that intersects earth's elliptical path. And since orbits are periodic, that payload is going to be intersecting earth's orbit regularly, meaning that in this instance, the chance of the material coming right back into the atmosphere is quite high.
What I remember of his explanation was the point that if you launch a payload of material into space, you have essentially placed that payload in an elliptical orbital path, one that intersects earth's elliptical path. And since orbits are periodic, that payload is going to be intersecting earth's orbit regularly, meaning that in this instance, the chance of the material coming right back into the atmosphere is quite high.