The reason now...
Is mostly a tradition. You had a keyboard, you had a CPU on your desk (a "desktop"), and your green screen CRT sat on top of it. The tower moved off the desk, and the monitor grew bigger. That is why the Horseless Carriage became a carriage without a horse before it became simply a "car". Someone said, "Hey, this thing goes faster, and goggles are a pain. Let's invent a WINDSHIELD. Someone went, going that fast at night is dangerous. Let's invent headlights. Someone went, Now we've got this big windshield in front of us, and it sucks getting wet. Let's put a roof over it. Someone else went, what if you had windows on all sides you could roll up. Someone else said, "steering with a stick is kind of awkward... let's use a WHEEL"... The forum started to follow the function.
It certainly isn't ergonomics.
I bet technology itself plays into it. With CRTs, it wasn't a practical viewing angle. You had the big old flyback transistor sticking out of the back of the CRT... making it go downward would have lifted the CRT from the desk awkwardly compared to the face of the screen. When we moved to LCD, the problem was initially viewing angle. By the time we got those both figured out, *custom* was so ingrained that no one has *thought* about it yet... Here is another million dollar idea I'm giving away on Tech Republic.
It is also a matter of it will feel *weird* at first. I really think I'm onto something. All these companies making "All In One" touch-screen desktop units that haven't caught on... make the monitor tilt radically from the bottom toward the user, raise it just a bit, give it good viewing at that angle, and have the keyboard on a tray that slides out a little but mostly has your hands *under* the display. I can imagine it right now, and it would be *awesome* sauce... *especially* as touch-screen.
