Why? Because 64-bit allows the game to access more than 4GB of RAM. Hollow Knight, while appearing "simple" due to its 2D hand-drawn art, loads massive interconnected zones into memory simultaneously. On a 32-bit system, the game would likely crash due to memory limits.
"Analysis of 2D Game Engines and Development of a Cross-Platform Game" hollow knight 32 bit
Even with 32-bit support, the game's heavy use of hand-drawn textures, particle effects, and Unity-based lighting can lead to frame drops and sluggish performance on older, low-end hardware. 3. Unofficial Ports and Community Efforts hollow knight 32 bit