为什么我不能使用DataTrigger来设置TextBox.IsEnabled = True？

In my application, I have a TextBox that I want to enable/disable based on an enum in my datacontext. The enum has three values (Anyone, Me, Someone) and I want to enable the Textbox when the value "Someone" is set. I am able to hack a solution by setting the value in reverse (see below). However, can someone please explain why the first solution didn't work?In my application, I have a TextBox that I want