Why did USB win out over parallel interfaces?
At face value sending 8 bits at once seems faster than sending just one at a time. In reality, there are other factors that affect parallel transfers.
The Achilles heel of parallel is the wait time between bytes. When the transmitter places a set of 8 bits on 8 lines it takes those bits a non zero amount of time to arrive at the receiver. Furthermore, those bits do not arrive at the same time. There is a spread between the fastest bit and the slowest bit. This spread requires that there is a blanking time between when the bits are asserted and when the bits are read.
At slow speeds this blanking time is irrelevant. As the speeds increase this blanking time becomes significant. The blanking time has to account for the worst-case scenario. This includes the cable, connectors, circuit board traces, etc... As speeds increase the blanking time will become the limiting factor.
With serial, all the bits come down the line back to back and require no blanking time. A speed increase only really requires the ability to cram the bits down the line faster. I'm glossing over that with serial signal integrity still needs to be maintained. Serial is also less sensitive to cabling abnormalities.