The problem is anti-aliasing.
It may be tempting to believe if you create a large icon in Illustrator with snap-to-pixel enabled, then it will scale down to any size while staying sharp. But horizontal and vertical edges in your artwork that lie on odd pixels – take pixels 5, 23 and 99 as examples – will, once scaled 50%, be rendered between pixels at the new size (ie. pixels 2.5, 11.5, and 49.5). Of course, there’s no half-pixels, so the edge is represented with an averaged colour, and blurriness is the result.
The only way to retain crisp edges is to only use pixel boundaries in the source artwork that will still fall precisely on pixel boundaries after scaling.
In the example shown below, the source icon (on the left) shows the red box drawn on pixel boundaries 3 and 29, and the white box on 8 and 24. When scaled 50% (right) only the white box remains crisp. The red box is at 1.5 and 14.5, so becomes anti-aliased.