Why DisplayPort, HDMI port etc. while USB can do the job?

I've been using HP EliteDisplay E232 connected to my hp notebook through DisplayPort for some time now. I was so glad that I changed to DisplayPort from VGA cable because the display is much better and crisp through DisplayPort.

I recently started using "HP port replicator 3005pr" for ease of connecting multiple devices to the notebook. Now I can connect headset, mic, network, keyboard, mouse, display etc. to the notebook through a single usb connector.

notebook -> usb -> port replicator -> other devices through their own ports

It's very easy and awesome. Interestingly I am finding the display as good and crisp as though the monitor is directly connected to the DisplayPort of notebook.

Now coming to the question - The video signal from my notebook is going through the USB out. That means it can transfer good data. If USB can transfer/deliver signal as high quality as the DisplayPort then why DisplayPort? Why invent all these multitudes of port specifications (HDMI, Display, Firewire etc.) and their versions? Why not every device come with a USB port and be super compatible with every other device?

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