Oh, Germany did - see for example the Asse II mine.

It just turned out that they weren't careful enough, so now they have got a giant nuclear waste storage pit which is unstable, is trying to leak into the groundwater, needs constant babysitting to prevent it from getting even worse, and will eventually need a nearly-impossible multi-billion-euro cleanup effort. At which point they'll be left with the original waste, plus a large amount of contaminated salt mine material, sitting above ground right where it started.

I reckon they would rather not want a repeat of this.