Because you keep your friends close and your enemies closer (or, as I tell my children, never turn your back on a vicious dog). If you put highly toxic waste in a volcano you can't be sure where it's going to end up. What if the volcano erupts and spreads your toxic waste over the whole planet?

If you're thinking of the volcano as an incinerator, then it would surely be more useful to incinerate the waste in an electricity generating plant? But that's a political issue these days.