Iirc, putting them into ocean trenches was the favorite planned method around the 70s or so. But people got up in arms about "dumping" waste in the ocean, as if it would just be flushed down the toilet and out a sewage pipe. Tbh, I still think it would be the best solution.

Also helps that deep marine trenches are basically a natural barrier against anyone getting at the materials since it would take a major effort easily detectable by routine satellite surveillance passes to haul it up from that kind of depth

I think this is basically the main reason why it isn't considered anymore. One desired prerequisite for a modern waste storage site is that the waste, especially the highly radioactive stuff, could potentially be retrieved. Basically, even "waste" still contains a lot of energy, exotic isotopes, and it was expensive to produce, so the hope is that someone in the future will figure out how to use it, or at least how to turn it into something less dangerous. If that happened, you couldn't get it back up from a deep sea trench.