In short - launching heavy stuff to space is expensive and nuclear fuel is heavy (elements) pretty much by definition. Also even the most modern rockets still

In short - launching heavy stuff to space is expensive and nuclear fuel is heavy (elements) pretty much by definition. Also even the most modern rockets still fail from time to time and you nuclear waste being spread over a wide area by an explosion or unplanned atmospheric reentry is much worse than having a few guards watch an heavy duty inert fuel container four a couple decades. And lastly today burned out fuel is still about 95% usable if you care to remove the impurities preventing further chain reaction that come from fission products. Not to mention all sorts of valuable materials that got created due to transmutation of part of the material via neutron capture in the reactor core.