It's due to the SEC. The SEC places restrictions on who can buy shares in the private startups -- eg. you need to be an "accredited investor" to purchase them (>$1M liquid assets or >$200k/yr earnings). The SEC also places limits on the number of shareholders a company can have without (effectively) going public... which creates an incentive for startups to disallow current/former employees from selling their shares on the semi-private market.