system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is usb a serial port and not a parallel port?
gpt
It is serial because the transfer speed that was required could be managed with just two data lines with differential signaling.   It also has two wires for power so a total of 4 wires.  
If you can have fewer wires you can make cheaper connectors and cables.  USB is designed to be a way to connect stuff.  
This is the main reason that USB is series because it will be cheaper.

Later USB standard add extra wires for dor signaling so USB 3.0 have2 additional differential pairs and a signal round for a total of 9 wires. The extra wires allowed higher transfer speed.

USB-C has 23 pins connectors but only 16 wires and a shield in the cable. The difference is because some pins are duplicated do the connector can be flipped.  
It uses 6 wire pairs for singaling.

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Even if USB 3.0 and the USB C  has multiple pairs of wire for signaling they still operate as serial connection side by side.  
A parallel connection would require the same timing on all wires and you would need an extra clock wire.  It also requires tighter tolerances for the wire and connectors.  
Using them as a separate serial connection that is self-clocking is a simple way to obtain high speed.

Parallel connection is simple to use if the speed you operate at is not limited by the performance of the wire but of the electronic, that drive it but it get harder and harder to do when you start to be harder and harder to do if you include signaling speed so you are closer to what is possible with the wire.