Why do cactus plants disintegrate when they get too much water?

A cactus plant disintegrates when it gets too much water, because of its tissues. Its tissues aren't set up to work with continuous or large amounts of water. Instead, they're adapted to get as much of the infrequent and insubstantial amounts of water that they can from a bright, drab, dry, extreme, harsh desert environment. Overwatering and waterlogging cause cactus tissues to blacken, rot, and die.