HDMI came from the TV manufacturers and was earlier than DP. While DP came from VESA and Computer OEMs.

HDMI being in TVs gave it a far wider penetration in the consumer market, and so when people wanted to hookup their laptops and other devices to TVs, they'd need HDMI.

Ironically, as ports have been simplified to almost just USB-C on many devices, DP's market share actually grows as it's cheaper and easier to include for OEMs, and if the consumer has to buy an adapter anyway, it might as well be on their dime to pay for HDMI, rather than the phone or laptop maker.

HDMI came around four years earlier than Display Port, so one of the reasons is that it has been around longer, therefore allowed mass adoption, first mover advantage.