Because it's a lot cheaper to bury it in bedrock, and it's just as safe. The scenarios in which underground storage lead to contamination are borderline hyperbolic: some sort of collapse leads society to lose all records of waste storage locations, some future society miraculously decide to dig right where the waste was buries, and lastly said future society is too dumb to realize the waste is poisonous (yet advanced enough to dig through a kilometer of bedrock).
And the point remains, what is the likelihood that some future civilization is going to happen to dig in the _exact_ spot where the waste was buried? And, if they're capable of tunneling through a mile of bedrock, wouldn't they have the smarts to notice that the stuff in those casks was poisonous?
