Because we fucking well could. We had a shiny new toy and we wanted to play with it. The things we were learning were supposed to usher in a brave new future and in our arrogance we thought we understood and control them. "Oh! It's perfectly safe to explode one of these things in the Van Allen Belt! After all, it's Clean, Atomic, Energy!" That little mess in the pacific pretty much put an end to our fun and games, though. We told the people involved in all that it was safe. A lot of them died of cancer. Oh well, you want to make an omelette, ya gotta break a few eggs.
My favorite story from back then was about the demon core. Google it.