The concern is that no matter where you put radioactive wastes, or how you contain them, they may leak out at some future time, and may dissolve in ground water and be carried away to a point where people will become exposed to them, which is a health risk since radiation is a cause of cancer. Most garbage is not as dangerous. Rotting organic matter is nasty and smelly, and can carry dangerous germs, but it is going to eventually be consumed by bacteria and other decay organisms, and it will effectively be recycled into the ecosphere. Plastic hangs around for a long time and does not recycle easily, but it isn't toxic, either. There are also various other forms of toxic waste, some of which are also very dangerous, but they can be chemically treated to make them non-toxic (or less toxic),