DisplayPort still doesn't have anything as good as CEC. DDC/CI exists and bizarrely enough has existed for much longer than CEC (since the DVI port era), but DisplayPort Alliance has never bothered to standardize it and iterate on it.
For example, some monitors crash if you read any value from the monitor, so you can only blindly send brightness or volume levels. Some internally use 255 instead of 100. Some have crappy flash and you will wear it out by sending values constantly. Etc, etc.
Yet HDMI is more widespread and both cables and equipment tends to be cheaper, which is surprising considering a USB-C to HDMI dongle needs actual hardware, while its basically just passthrough for an USB-C to DP.
It's also quite nice that HDMI keeps basically all the logic and signaling the same as VGA (blank periods, EDID etc.), so actually making use of the signal is much easier.