It all stems from the companies behind the HDMI authority. It's basically all of the major AV device makers circa early 2000s. They wrote the spec and added it to all of their products. Displayport wasn't around just yet so HDMI just beat it to market. Since everyone needed an HDMI thing to go with their HDMI thing, everyone else jumped on the HDMI bandwagon. Although I'm really not sure how HDMI managed to get it's way into PCs. Displayport should have just cornered the entire market, it's very popular on business-class machines. I'm guessing it's because of HTPCs and people wanting to put big TVs on their PCs is what led to the adoption.

I think the HDMI connectors popped up at the same time screens switched from 16:10 (VESA compatible at the time) to 16:9 to be more cost effective for the manufacturers. But I’m not sure why. I looked at graphicscards and wondered why HDMI suddenly gained traction in the PC space even after the release of DisplayPort. I think this should never have happened.