Didn’t the Department of Energy try this and why didn’t it work?

The Department of Energy tried to start a test program for vertical boreholes intended for defense waste (such as the waste stored at the Hanford facility in Washington State or the Savannah River facility in South Carolina). This program envisioned vertical boreholes even deeper than ours, drilled into crystalline basement rock such as granite. The founders of Deep Isolation were familiar with this program but felt that there was a great advantage to drilling into less brittle rock and taking advantage of directional drilling.