HDMI is not much bigger than mDP and doesn't add thickness over USB ports which already exist on those laptops. OTOH, it was more commonly supported by non-Appl

HDMI is not much bigger than mDP and doesn't add thickness over USB ports which already exist on those laptops. OTOH, it was more commonly supported by non-Apple monitors in that time and very easily adapted to the ubiquitous DVI-D (same protocol and cabling, different plug). If Apple cared about interoperability and user convenience (no silly dongles, yay) they would have used that.