Answer
The endodermis provides a hydrophobic barrier in the endodermis— all substances must enter the cytoplasm to cross the endodermis. The inhibitor must be unable to do so and, thus, stays in the roots.
Work Step by Step
Scan Concept 36.3 for information about the transport of water and look for any mention of barriers between the roots (entry of the inhibitor) and the leaves (where it should act). Try to imagine a way in which any barrier could stop the movement of the inhibitor to the leaves.