Answer:
Because of diversion and climate change
Explanation:
The Syr Darya and the Amur Darya have been diverted due to extensive irrigation projects, that later failed. One of the Amur Darya's sources, the Zeravshan river, formerly reached the Amur Darya. The problem worsens further due to climate change, which evaporates the water.
edit: Clarification, the Syr Darya and the Amur Darya are the Aral Sea's sources
