Having been at Apple, and also at a large defense contractor, where I would routinely walk by areas that said "Top Secret Clearance Required", I can say that Apple simply put more effort into making secrecy part of the culture. A few things I noticed at Apple:

• code names for every product - no one referred to products any other way
• team members who are also on "special" teams don't tell their coworkers what they do
• black curtains and frosted windows
• trash bins were monitored

The higher ups at Apple were more willing to spend money, time, and effort to protect information. Plus, as other posters mentioned, incentives such as camaraderie and fear played into things as well.