Because every time I make a suggestion at work my CEO yells at me, ridicules me, and makes me feel dumb for speaking up. Then he ends up thinking about it and realizng it's a good idea. Do we really need to go through this EVERY time?
Document your suggestions in writing, and then document what happens as a result of your speaking up. They'll come in handy when it's time for your performance review.
Ammo, baby, ammo.