They just dumped them there in the 60s and 70s before there was regulation...

Edit:
Here's two links, if you want to read up on it:

• English Wikipedia: https://en.wikipedia.org/wiki/Asse_II_mine
• Most official source you will find (German): https://www.bge.de/de/asse/themenschwerpunkt-radioaktive-abfaelle/
  (Mind that a translator will like translate the name of the mine "Asse" as "Aces".)