probably because the early adapters of high-res in consumer markets (TVs, game consoles) were using HDMI in the first place (PS3 and XBOX 360). Displayport is more prevalent in PC use (not even laptops except macbooks which uses mini-dp).
I'm pretty sure most dell laptops have mini DP. My desktop grapics card also has two mini DP ports which I used with a DP to DVI cable without adapter. (something like https://www.amazon.com/SIIG-DisplayPort-Converter-Cable-CB-D...) You need an active adapter if you want dual link dvi though.
The most recent graphics cards (1080/1070) have 3 DP ports, one HDMI, and one DVI. Also, every laptop I own/use/know of has a mini-DP port too, across roughly 4 brands.
HDMI got their foot in the door first, but I think we'll be seeing more DP stuff in the very near future. The only downside of DP is the cables are usually more expensive.
