Because of the countercurrent multiplier system in the loop of Henle and the proximity to it of the medullary collecting ducts,

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The countercurrent multiplier system in the loop of Henle generates a hyperosmotic medullary interstitium, which drives the passive diffusion of water out of the descending limb and into the ascending limb. The active reabsorption of sodium in the thick ascending limb further contributes to the generation of the high osmolarity in the medullary interstitium. When the hyperosmotic fluid enters the medullary collecting duct, water can be readily reabsorbed in response to vasopressin, which results in the generation of concentrated (hyperosmotic) urine.

Because of the Countercurrent Multiplier System in the Loop of Henle