Actually, while you will often hear the "launch vehicle could explode and then radioactivity would be dispersed everywhere" argument, I don't think it's actually a valid argument.
Look at the space-based RTGs we've been using ever since pretty much the beginning of space exploration. They have the radioactive material packaged up in a container which will survive the most extreme conditions, including catastrophic failure of the launch vehicle and even atmospheric reentry without releasing the radioactivity.
So clearly if we wanted to shoot radioactive waste into space it is possible to do so with no danger.
The real reasons are simply that it would be incredibly expensive and it's just not necessary.
Radioactive waste is not a huge drama or problem like some people imply it to be.
There's not that much of it, it's being stored safely, most of it is useful material, and it hasn't ever hurt anyone.