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

I have a Macbook Pro coected to a Dell P2714H IPS 27-Inch monitor. It coects with a mini display port to display port cable. (The monitor has only a large display port coector, it does not have a HDMI coector.) 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 coect 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?