Traditional parallel interfaces, where all signals were timed off the same clock, suffered from "skew": signals arriving on different wires at different times. This made it difficult to continue increasing the speed of parallel interfaces, particularly those that operated over longer distances.
As a result, serial interfaces with clocking embedded in the data stream rose in popularity. Parallel printer ports gave way to USB, PATA gave way to SATA, SCSI gave way to SAS.
However there is a limit (which increases over time as technology improves) to how fast a serial interface can be with affordable transceivers and wiring. The solution to this are what are known as "multi-lane serial" interfaces. Each "lane" has its own timing, but the data streams are split at the source and merged back together at the destination.
Multi-lane techniques have allowed serial interfaces to not only compete in, but come to dominate the high speed world. A single lane of PCI express 1.0 could outperform regular PCI, but not the faster variants of PCI-X or AGP. On the other hand with 16 lanes PCI express could easily outperform PCI-X and AGP.
USB was originally designed to replace a bunch of PC peripheral interfaces, none of which were particularly fast. The priority was to be cheap; people wouldn't switch to USB devices if they were significantly more expensive than their traditional counterparts. A single half-duplex serial link was the order of the day. It got a speed bump with USB 2 to better support external storage.
With USB 3, the original design had run out of steam and the solution was essentially to put two entirely independent interfaces on the same connector: the original bidirectional low/full/high speed data pair and two new unidirectional data pairs to carry the "superspeed" traffic. USB 3.1 bumped up the data rate on the "superspeed" pairs.
USB C was created to be "one connector to rule them all"; in addition to supporting all the existing USB functionality, it was also designed to carry a variety of "alternate modes" including DisplayPort, HDMI and Thunderbolt. The connector was also designed to be reversible and to support delivery of substantial amounts of power. The alternate modes necessitated having four high performance data pairs. There were also two "sideband use" pins to support the alternate modes and two "configuration and control" pins, which are used to detect, negotiate alternate modes, and negotiate power delivery.
So it all comes together, USB C had four high-performance pairs (plus the legacy pair) but two of them weren't actually used in USB mode. Furthermore thanks to the reversibility, superspeed devices with USB C ports were required to have either multiple transceivers or signal switches to direct the superspeed signals to the correct pins depending on cable orientation. Multi-lane serial was a proven technology (PCIe had been in widespread use for over a decade). The natural thing to do was to add a two-lane mode to USB, doubling throughput at little extra cost.