At first, I thought the "cost" might be a factor (i.e. cost of installing them everywhere at once), but I believe "The Rodent" is rich enough to get that done. Maybe... just maybe... it's a matter of "demand." I mean, where are those security measures in the most demand within the entire complex? What part of the park imposes the greatest risk and, therefore, demands those measures? IMHO, that would be the "Magic Kingdom" and thereabouts, because that's where the greatest concentration of kids are. I think it's also the most well known of the complex's components and, therefore, the most recognizable target. The 9/11 terrorists attacked both United and American Airlines because of their size and notoriety... not "Swiss" Airlines or "Alaska" or "Southwest." Furthermore, even though they could probably install them everywhere, I also wonder if they're reacting to public backlash over being searched all over the place and, therefore not installing them everywhere where the "demand/risk" is not as high.