Can't convert HDMI to DisplayPort - root cause?

HDMI and DisplayPort are completely different from the point of view of the signals that they send down the wires, as well as the connectors that they use. However, a DisplayPort output has the ability to detect when it's connected to a HDMI display, and send HDMI signals down the wire to that display. This only requires a passive adapter or cable with the right connectors on each end.

The reverse isn't true because HDMI is an older standard, and such a capability has never been added to the standard in the meantime. A HDMI output port can't send DisplayPort signals because it doesn't know how to do that, and it's not required to.

HDMI is a clock-less video format, DP uses a clock to keep video in sync.

DP adds the clock to the video at the sending device; so a DP-receiving device needs to see a clock on the incoming video, HDMI doesn't. This is why it's very easy with a simple adapter to go from a DP output on a laptop to a HDMI input monitor because the monitor doesn't care about the clock.

When going from HDMI to DP, the DP-receiving device is looking for a clock; HDMI sender does not have one, so this does not work. You need an 'active' adapter in this case and all that adapter does is add a clock to the video format and sends it out via DP. Notice I said "active" adapter--that's the magic word you need to look for when getting an adapter to go from HDMI to DP.

It has absolutely nothing to do with the age of the video format (as another user put it "older standard"). HDMI 2.0 is newer than DP1.2 and the issue would still exist for the very reason I stated above.