Think of things like I2C and the IBM PC Keyboard interface.

A UART effectively has to extract a "clock" from the received data, so that it can identify where the individual bits lie; providing a clock on the interface simplifies this, and making the data line bidirectional means you still only need 2 wires.

Note that the RS232 standard does provide for synchronous communications (the 'S' in "USART"), but this is rare on PCs, and not supported at all on the 9-Way connectors.