Yucca Mountain was a bad site. Once they started digging they found that the ground was too loose. It wouldn't be able to support the weight without sinking. Have you ever seen the foundation of a house that sank on one side? The concrete buckles as the weight of the house slowly compacts the soil. The same thing will happen with millions of tons of waste, steel, and concrete. It's why missile silos were built in bedrock, not loose soil. Not to mention the technology wasn't going to allow digging deep enough to store all that much. It would mostly be used for waste from nuclear weapons, ship reactors, and other military projects. Not really that much space would have been available for commercial power generator use.

The conservatives who pushed for it did it because the contractors paid them to. It was blocked because the waste would leak not in thousands of years but in maybe decades. Not to mention the land was stolen from Native Americans and they didn't want nuclear waste in their stollen land. Among many other issues.

Edit: besides the Clintons have always been conservatives, too. So they're in that bucket. They're just moderates.