as an ex-user of 5 gallon buckets, the answer is simple. i used them for fired brass, 40-80 lbs per bucket, maybe owned 80 buckets.
its lack of ventilation. they fit so well they have no place for air to bleed back in as the come apart. as nt good were large and dry, we would drill a small hole in the bottom of each one...presto bango..no sticking buckets