A single wire transmits the bits one after another. Some systems for local communications send multiple bits at a time. Data transmitted between a PC and a printer is sent 8 bits at a time over a printer cable. Most networks, particularly long distance connections, send data one bit at a time. Over long distances there can be a time skew problem where all of the parallel bits do not arrive at the final destination at the same time.
Cost is also a major consideration.
A long cable with enough conductors to send 8 parallel bits costs more than a cable to send one bit at a time. Data can be sent synchronously or asynchronously. With asynchronous communications, the sending of a byte does not determine the time the next byte will be sent. With synchronous transmissions there is a time dependency between the bits or bytes being sent. Bits can be synchronized so that each bit is sent at a specified time after the previous bit.