This is a difficult one to get your head round. It was for me anyway.
A cork will normally float to the top of a stationary bucket of water as the relative density of the cork is less than the relative density of the water. Density is simply a mass per unit volume. The cork is therefore buoyant, and the force of this buoyancy will cause the cork to rise to the top of the water.
When stationary, the bucket, water and cork are all subjected to gravity in the same way, and their relative densities will cause the cork to rise.
When dropped from the tall building, the bucket, water and cork are effectively under freefall conditions, and therefore weightless for the duration of the fall to the ground.
As the water and the cork are weightless, their relative densities play no part in the problem, and therefore the cork has no buoyancy force within the water and cannot rise.