They didn't speak up because:
- They had not had a chance to study the subject and so felt that their comments might be out of line.
- They had studied the subject and liked the proposed action.
- They had studied the subject and thought the proposed action was a mistake but they felt the timing was not right to surface their objections.
- They thought the proposal was unwise but didn't think the matter was a battle worth fighting.
- They thought the proposal was unwise but hoped to gain its proponents as allies on another project.
- They thought the proposal would do no serious harm.
- They thought that the points they would have made had already been made, possibly in a more eloquent manner.
- They feared retaliation.
- They feared that their reputation might be harmed.
- They were willing to speak but they wanted to be called upon.
- They simply didn't care.