There is a certain problem that the Illuminati and the MKUltra folks have encountered in their quest to perfect the enslavement process that they use. It is simply this. Some people are totally immune to it, their core personalities do not allow the external programming to hold.
They can still try to program these people, but as soon as the programmers are done, the programming starts to fade. The barriers between personas fade quickly, and even though the mind is shattered, it begins to heal itself. The triggers fade and cease to be effective. These people rearrange their mental fragments to suit their core personality.
The one thing that disrupts MKUltra is when a person's core personality is fundamentally altruistic. Now in the case of Diana, they may have missed this because they were trying to create a charismatic brood mare who would make the people love the royals. So they wanted someone who seemed altruistic and probably did not notice that she was slipping from their grasp.