The Demogorgon took Will Byers primarily because Will was near the gate to the Upside Down when Eleven was using her powers, which attracted the creature to him. Unlike other victims who were often bleeding and thus attracted the Demogorgon, Will wasn't killed right away but was taken, likely because he was specifically targeted as part of a larger plan involving the Mind Flayer. Additionally, Will's proximity to the Upside Down gate and Eleven's use of her powers in the area made him vulnerable to being found and taken by the Demogorgon. Some fan theories also suggest deeper connections, such as Will becoming a vessel for the Mind Flayer or even transforming into a Demogorgon- like being in future timelines, which explains why the creature left him alive instead of killing him immediately.
In short, Will was taken because he was caught in the wrong place at the wrong time near the Upside Down gate, influenced by Eleven's powers, and targeted by the Demogorgon for a specific purpose beyond just predation.
