Mass-market compatibility.
It's already difficult to find TVs with four fully-compliant HDMI ports; often you'll get a TV with one HDMI 2.1 port and three HDMI 2.0 ports, and sometimes the 2.1 port will also be the only eARC port so you have to choose between high framerates/resolutions and using a sound bar. In other words, even with just HDMI getting a decent set of ports is difficult.
The idea of TV manufacturers also adding DisplayPort ports seems ludicrous to me - not because it's a bad idea, but because I can't imagine them going to the trouble if there's no tangible demand. At best I could see them replacing HDMI ports with DP ports because there's limited space on the motherboard, but that would still require the board to have both HDMI and DP circuitry/chipsets and HDMI/DP certification/testing.
Then you have a TV with, say, two HDMI ports and two DP ports - which, for most users, means "two ports" since 99% of people don't have any hardware they want to connect to their TV that supports DP anyway.
So basically unless we start seeing game consoles, AppleTVs, and Rokus supporting DisplayPort we won't see TVs supporting DisplayPort, and we won't see any of those devices supporting DP because they don't need to - HDMI works fine for them and it's sufficiently universal.
Maybe China's new HDMI replacement will take off over there and make its way into devices over here, but I'm not holding out hope.