The issue is that those "acres" are for short-term storage, and the waste that's in short-term storage isn't in a form that's safe for very long. Typically it looks like a swimming pool, and the water helps cool the waste. You don't want that water to disappear (earthquake? terrorism?) and you don't want someone to grab some (suicide, yes, but we've seen that before, and this is a great way to get a dirty bomb going).
Long-term you need deep storage, such as Yucca Mountain, deep salt mines, etc... but you also need that storage to be in a form that still shouldn't leak - you can't just chuck it into steel or poly barrels and hope for the best.
My recollection - and I haven't read about this in at least a decade or two - is that vitrification, where the waste is essentially mixed with molten glass and the waste is part of the resultant matrix, is the most likely to avoid leaching; someone would have to chip off a chunk of glass to move the waste around and it would be the safest for adding to the storage facility because there's no worries about bringing in another load and finding you need a sump pump for that sludge that's suddenly on the floor
