Why High-Speed Buses Are Serial Rather Than Parellel:

In times gone by, people thought that parallel data connections were the way
to go for high speed. After all, they transfer whole bytes of data at a time,
rather than single bits. Surely that must mean they would be faster, right?

Yes and no. Unfortunately, parallel connections have been plagued in the past
with synchronization problems. It is difficult for the device on the other
end to get the bits in exactly the right sequence; They must all arrive at
the same time, or else the bytes they form get scrambled. Although parallel
connections work pretty well right now, they get very unreliable at higher
clock rates.

Serial connections, on the other hand, have no such problem. The device on
the other end simply receives the bits one by one, without having to worry
about synchronizing them; It just arranges them in the order it receives
them.

At the same clock rate, parallel connections are indeed faster, because they
send 8 times as much data in the same time frame (assuming 8-bit bytes).
However, it is possible to reliably raise clock rates much higher on a serial
connection than on a parallel one. This is why the new generation of
high-speed connection schemes (like USB and FireWire) are all serial.