Because you can't install any third party software on Twitter. Tracking how many people see your tweet would require some sort of tracking on Twitter.com and on each separate Twitter client available.
What you could show is the reach, or the number of users that your tweet is shown to. Based on your followers, and the followers of people who retweet or @mention you, you can come to an estimate of people who's timeline contains your tweets. This is, however, by no means an accurate statistic.