As far as I am aware, after having done exhaustive research on this, its licensing costs and popularity. Display port simply isn't popular enough. The vast majority of TV manufacturers (not brands mind you, many white label their manufacturing to different brands) also make monitors, and adoption of HDMI across both tvs and monitors not only was much higher, it was overall cheaper in cost since you could share the same components across lines. This being driven by cheaper licensing costs for accessory manufacturers (like blu ray players).

Its also easier to implement, if I recall correctly

This is the essential core of it, as I have come to understand it anyway.