Re:Speed (Score 5, Informative)

I wondered this too, so went digging. the most concise and, to me, most credible answer was the following (Credit to K. Adam's at Geek.com)

Serial Faster than Parallel... (5:41pm EST Wed Jul 25 2001) The problem with parallel (ribbon) data transfer cables is the crosstalk that occurs between adjacent conductors at very high clock/transfer speeds. IBM developed a work-around for ATA-66 and ATA-100 by using an 80-conductor cable with a 40-pin interface, by stringing a "ground" conductor between each "signal" conductor. Capacitance issues, "standing waves," and impedance (electrical resistance as relates to rapidly-changing voltages) matching problems become more evident in parallel (ribbon) cables as you crank up the clock/transfer speeds, also. It's a lot easier to match the impedance of a few conductors in a serial cable to its interface, than trying to match impedance for 40 conductors. Parallel schemes actually have a lot less "processing" overhead than serial schemes, but you're ultimately limited by physics a lot more quickly than with serial... - by K. Adams

Kacd.