The reason is because when you connect an external display, even if you close the laptop and use it in clamshell mode, the laptop automatically switches from an integrated video card (this uses much less power) to a more powerful, discrete video card (this uses a lot of power).
This is also true regarding Intel Iris Graphics, which has different power profiles (see the "Power Management" section). Also read on Embedded DisplayPort (eDP), and it's possibilities in terms of saving system power and further extend battery life in portable PC systems. I'm not the specialist in GPU-chips architecture, but my guess is, that the difference is exactly in using of Embedded DisplayPort on a mobile CPU, against of DisplayPort connections when on external monitor.
So the battery is drained more on when external monitors are connected, it needs more GPU power. Monitor itself does not drain the battery, as it's connected to external power.