Because there is no need to. The radioactive material wasn't a problem before it was dug out of the soil. It's not a problem if it returns to where it came from. The problem is that most government try to find an idiotic central storage location instead of simply distributing the spent fuel over many locations.
Yes. Launch vehicles sometimes fail. They may explode, or they may not make it all the way to orbit and have to re-enter. Either way, this would scatter the radioactive material over a wide area.