Why does a HDMI cable not work, but a Display-Port cable does work?

I have a Macbook Pro connected to a Dell P2714H IPS 27-Inch monitor. It connects with a mini display port to display port cable. (The monitor has only a large display port connector, it does not have a HDMI connector.) It works fine.

This is a close up of the above mini display port to display port cable. It works fine. I would prefer to connect with HDMI. The reason is so that I can put my HDMI switch in the middle, and be able to switch the monitor to the macbook or windows pc.

So I got a mini display port to HDMI cable + a HDMI to display port adapter. It does not work. It all seems to fit fine, but the monitor is blank.

I wondered if the adapter is bad, so I got another one. It still does not work. Any ideas?

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