The reasons are:
1) Usability: Using "Build / Run target selector" for switching the active project requires a bit of interaction, you need to:
a) click on the "Build / Run target selector" (or hit the relevant shortcut)
b) click on the Project combobox inside (or hit another relevant shortcut)
c) find the desired project on the list (couple of key up/down hits)
d) activate it (click or enter hit)
If we had a special action in the context menu of Projects view we would need much less of interaction (especially when we would provide shortcut for it).
2) If the active project is marked in the Projects view (with bold) the user expects he can somehow change the active project within this view. It's unintuitive that he needs to go to another place in order to change it.

Side note: after choosing that action the current project shown in "Build / Run target selector" should be updated accordingly.

General note: this is about changing the active project only, without changing the run target.