I remember reading while DisplayPort was new that it was a solution to getting graphics cards to output to more than two screens. AMD did a whole song and dance about EyeFinity with 3 and 6 monitor arrays that depended on the new DP standard to work. However it cost so much to set up (getting a stand to VESA mount 3 or 6 monitors was prohibitively expensive back then) that it never really got traction.
Back to the point, I think the reason why all modern graphics cards use miniDP ports (and maybe one DVI-I and/or HDMI) is because it's so easy to find adaptors for them to any standard your monitors/TVs use. They're all digital signals, and the main difference between the different standards are how they handle the signal (educated guess, too lazy to google up the actual answer) and available bandwidth (e.g. For UHD/4K 60hz).
