Because communication apprehension is the fear or anxiety that exist because the speaker thinks that the audience will not form a good opinion on speaker's speech. So the person that has fear associated with the jugdement of people will have to overcome it by excessive practice. After four to five speeches experience this fear vanishes because doing things that make you fear looses its frightening perception in mind which becomes normal at the end.
The person who has less communication apprehension do not have to practice much because it is normal for him.
